JPH10188332A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical head device whose miniaturization is easy and which has good productivity by allowing at least one side of liquid crystal cell substrates of liquid crystal to be arranged in between a beam splitter and optical recording mediums to have a projected part or a recessed part, allowing liquid crystal molecules being in the inside to be twisted and making a focus or the phase distribution of light beams variable by electrodes provided on the substrates to enhance the utilization efficiency of light beams. SOLUTION: When a voltage is not impressed on electrodes 14, 15 of substrates 11, 12 being at upper and lower sides of a liquid crystal lens, light beams of P-polarized lights emitted from a light source pass through the beam spliter of a polarization system and light beams which are made circularity polarized lights of a clockwise direction by a phase difference plate 3 pass through liquid crystal lens by almost without being diffracted to be focused on a first optical recording medium. The light beam reflected from the first optical recording medium becomes a circularity polarized light to pass through the liquid crystal lens as it is and it is returned to a linearly polarixed light with the phase difference plate. When the voltage is impressed on the electrodes 14, 15, light beams of P-polarized lights emitted from the light source are made the polarized lights of a clockwise direction and they are focused on a second optical recording medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for writing optical information on an optical recording medium such as a CD (compact disk), a CD-ROM, a video disk, and a magneto-optical disk, and for reading optical information. The present invention relates to an optical head device.

[0002]

2. Description of the Related Art Conventionally, in an optical head device for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk and for reading the optical information, a CD / CD-ROM is used.
In order to realize reading and writing of signals with respect to disks having different thicknesses such as a DVD disk and a DVD disk with a single optical head device, the following configuration has been adopted.

For example, a Fresnel lens type blazed hologram is formed on the surface of a lens, and for example, about half of the light incident on the lens from the semiconductor laser is diffracted by the hologram in the direction in which the beam spreads, and the other half is transmitted as it is. Then, by converging each beam by a lens body, light having two focal points is produced by one optical head device. It has also been attempted to make the lens the same as the conventional one, and separately separate and arrange a Fresnel hologram lens plate having the same function as above.

[0004] However, in these systems, the hologram reduces the amount of light on the forward path by half and again on the return path by half, so that there is a problem that the amount of light is reduced to 1/4 or less during reciprocation.

For this reason, in the case of an optical head device using a red semiconductor laser, for which it is particularly difficult to obtain a large output, the load on the light source increases, and the power consumption increases.
There has been a problem that the optical head device is increased in size, cost is increased, and reliability is reduced.

It has also been practiced to prepare two lenses having different focal lengths and mechanically switch and use them. However, since the lenses are mechanically moved and used, the size of the optical head device increases. However, there is a problem that the cost increases and the reliability decreases.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, improves the efficiency of light utilization, is easy to miniaturize, and provides an optical head device incorporating a bifocal lens which can be manufactured inexpensively with high productivity. For the purpose of providing. It is another object of the present invention to provide an optical head device incorporating a bifocal lens that can be used in a so-called polarization system using a polarization hologram or a polarization beam splitter.

[0008]

SUMMARY OF THE INVENTION The present invention relates to an optical head device having a light source, a beam splitter, a liquid crystal lens, and a photodetector, wherein a liquid crystal lens disposed between the beam splitter and the optical recording medium has a liquid crystal cell. At least one of the substrates has a concave portion or a convex portion, the liquid crystal filled therein is twisted, and the focal length or the phase distribution of light is made variable by electrodes provided on at least a part of the substrate. The present invention provides an optical head device characterized by using:

In an optical head device having a light source, a beam splitter, a liquid crystal lens, and a photodetector, at least one of the substrates of the liquid crystal cell has a fine concave as a liquid crystal lens disposed between the beam splitter and the optical recording medium. Or, it has a Fresnel lens structure with a convex portion, the liquid crystal filled inside is twisted, and the focal length or the phase distribution of light is made variable by an electrode provided on at least a part of the substrate The present invention provides an optical head device characterized by using:

Further, an optical head device in which the substrate at the center of the liquid crystal lens is substantially flat, and the ordinary refractive index of the liquid crystal is n _o , the extraordinary refractive index is n _e , and the twist pitch is P , When the wavelength in vacuum is λ, (n
_e -n _o) ² P / (to provide an optical head device which is 8.lambda) ≦ 0.05.

Further, an optical head device in which the refractive index of the substrate is substantially equal to the ordinary light refractive index or the extraordinary light refractive index of the liquid crystal or the average of the ordinary light refractive index and the extraordinary light refractive index, and a beam splitter thereof. And an optical head device using a polarization diffraction element in which an optically anisotropic material is filled between substrates using a substrate provided with a lattice-shaped concave portion as at least one substrate.

In the present invention, since a liquid crystal lens is used, the focal length or the phase distribution of light can be switched by applying a voltage from the outside, and an optical head device with high utilization efficiency can be obtained.

[0013]

FIG. 1 is a schematic diagram showing a basic configuration of the present invention. In FIG. 1, 1 is a light source such as a semiconductor laser, 2 is a beam splitter, 3 is a phase difference plate, 4 is a liquid crystal lens, 5 is a condenser lens, 6 is a first optical recording medium, 7
Denotes a second optical recording medium, and 8 denotes a photodetector.

Light emitted from the light source 1 is transmitted to a beam splitter 2
Passes through the phase difference plate 3, passes through the liquid crystal lens 4, and is condensed by the condenser lens 5 to reach the optical recording medium.
Here, whether or not a voltage is applied to the liquid crystal lens or the applied voltage is changed to change the focal length of the liquid crystal lens or the phase distribution of light to change the first optical recording medium 6 or the second optical recording medium. Focus on 7. In the present invention, the beam splitter may be a prism-shaped one or a polarizing beam splitter such as a liquid crystal hologram.

The light reflected from the optical recording medium and returned again passes through the condenser lens 5, the liquid crystal lens 4, the phase difference plate 3, and the beam splitter 2 again, and the light separated by the beam splitter 2 is converted into light. It reaches the detector 8.

FIG. 2 is a sectional view showing an example of a liquid crystal lens in which a substrate has a concave portion or a convex portion. In FIG. 2, 11,
12 is a substrate, 13 is a recess provided in the substrate, 14,
Reference numeral 15 denotes an electrode, 16 denotes a peripheral sealing material, and 17 denotes a liquid crystal filled between the substrates.

As the substrates 11 and 12, a transparent substrate such as plastic or glass can be used. A concave portion or a convex portion is formed on at least one inner surface side (liquid crystal side) of the substrate. In this figure, a concave portion is formed on the substrate 12 side. The concave portion or the convex portion may be formed on the substrate itself, or an organic or inorganic transparent film may be formed on the surface in a predetermined shape.

When this processing is performed on the substrate itself, it may be formed by mechanical shaving, press molding, or etching. When forming an organic or inorganic transparent film on the surface, after forming the transparent film on the entire surface, it may be formed by shaving or etching as in the case of the substrate itself, and may be directly deposited in a predetermined pattern. It may be formed by printing or printing.

FIG. 3 is a sectional view showing an example of a liquid crystal lens in which the substrate has a Fresnel lens structure. In FIG. 3, 2
Reference numerals 1 and 22 denote substrates, 23 denotes an uneven portion of the Fresnel lens structure provided on the substrate, 24 and 25 denote electrodes, 26 denotes a peripheral sealing material, and 27 denotes a liquid crystal filled between the substrates. The concave and convex portions of the Fresnel lens structure can be formed by the same method as the method of forming the concave or convex portions on the substrate.

The irregularities of these lenses may be completely formed into a predetermined shape, and there is no problem even if only the central portion is used in a flat shape so as to facilitate processing. In particular, in the case of a Fresnel lens structure, it is preferable that the center portion be flat to facilitate processing. The central portion means a region inside the diameter of about 20 to 60% with respect to the outer diameter of the lens.

The electrodes 14, 15, 24 and 25 are formed of a normal I
A transparent electrode such as TO can be used. Normally, the entire surface may be a solid electrode. However, for example, it may be patterned in a ring shape to partially change the lens function. Further, the resistance can be reduced by providing a metal wire or the like in a part.

Although not shown, an alignment film of polyimide, polyamide, SiO, or the like is formed on the electrode and used. In a typical example, a polyimide film is formed, and the surface is rubbed to form an alignment film. The rubbing direction of the alignment film can be used depending on whether or not the liquid crystal is twisted between the two substrates.

The two substrates thus formed are arranged so that the electrode sides face each other.
Then, liquid crystal 17 and 27 are filled inside. As this liquid crystal, a normal nematic liquid crystal is used.

Next, the operation of the optical head device will be described. It is assumed that light emitted from the light source 1 has linearly polarized light, for example, P-polarized light (polarized light in a direction parallel to the paper surface). A nematic liquid crystal of positive dielectric anisotropy with the liquid crystal lens of FIG. 2, an intermediate value between the refractive index of the substrate 12 and the ordinary refractive index n _o and extraordinary refractive index n _e of the liquid crystal (n _o + n _e) / The rubbing direction of the alignment film on the light source side is set to be parallel to the paper.

In this case, assuming that the liquid crystal is twisted at a twist pitch P (360 ° twist pitch) with right-handed twist, the effective refractive index of the liquid crystal with respect to clockwise circularly polarized light is approximately (n _{e + n o) / 2 + (} n e -n o)
^It is expressed as ² P / (8λ). Further, the effective refractive index of the liquid crystal with respect to light of left-handed circularly polarized light is approximately (n _o + n _e)
/ 2- (n _o -n _e) is expressed as ² P / ^(8λ).

When the liquid crystal lens 4 is in the off state, the liquid crystal in the substrate on the light source side is oriented in a direction substantially parallel to the substrate and parallel to the plane of the drawing. On the opposite substrate (optical recording medium side),
For example, it is assumed that the substrate is oriented at an angle different from the orientation angle of the substrate on the light source side such as a 90 ° twisted state.

On the outward path, the light emitted from the light source 1 passes through the beam splitter 2, is then converted to clockwise circularly polarized light by a phase difference plate 3 such as a λ / 4 plate, and enters the liquid crystal lens 4. The beam splitter 2 is a polarization beam splitter that functions or does not function as a beam splitter depending on the polarization direction of light.

[0028] At this time, when the _{(n e + n o) /} 2 in than ^{_{(n e -n o) 2 P}} / (8λ) is small, the liquid crystal effective against light right-handed circularly polarized light refractive index is substantially equal to approximately _{(n e + n o) /} 2. For this reason, in the outward path, the light emitted from the light source 1 has a refractive index of the substrate (between the ordinary refractive index of the liquid crystal and the extraordinary refractive index of the liquid crystal) and the refractive index of the twisted liquid crystal which are substantially equal to each other. Since they are equal, light travels almost straight without being refracted. Then, the light is condensed by the condenser lens 5 and focused on the first optical recording medium 6.

On the return path, the light reflected on the surface of the first optical recording medium 6 becomes counterclockwise circularly polarized light, passes through the condenser lens 5 and the liquid crystal lens 4 not functioning as a lens again, and The light is returned to linearly polarized light by the phase difference plate 3, is separated by the beam splitter 2, and reaches the photodetector 8.

In the present invention, it is important that the effective refractive index of the liquid crystal portion for clockwise circularly polarized light and the effective refractive index of the liquid crystal portion for counterclockwise circularly polarized light are substantially equal within a practically allowable range. Become. For that purpose, the pitch P is preferably not so large. Specifically, the pitch P is 5 μ
m, particularly preferably 3 μm or less.

When the ratio d / P between the pitch P of the liquid crystal and the thickness d of the liquid crystal layer exceeds 1.0, light scattering is caused by the focal conic state in which the liquid crystal helical axis is disturbed when the voltage is turned off. Turn-off time tends to increase.
For this reason, it is preferable to reduce the viscosity of the liquid crystal, or to increase the angle between the liquid crystal alignment vector near the substrate interface and the substrate surface, that is, increase the pretilt angle.

When a voltage is applied to the liquid crystal lens 4 and the liquid crystal lens 4 is turned on, the liquid crystal is aligned in the direction of the electric field, and is oriented substantially perpendicularly to the substrate (up and down in the drawing). For this reason, on the outward path, the light emitted from the light source 1 passes through the beam splitter 2, and is then converted to clockwise circularly polarized light by the phase difference plate 3,
Incident on.

Here, the refractive index of the substrate (between the ordinary light refractive index of the liquid crystal and the extraordinary light refractive index) and the refractive index of the liquid crystal (which becomes the ordinary light refractive index) do not match, and thus function as a concave lens. Light is refracted. For this reason, when the light is condensed by the condensing lens 5, the focal length increases, and the light is focused on the second optical recording medium 7.

On the return path, the light reflected on the surface of the second optical recording medium 7 becomes left-handed circularly polarized light, passes through the condenser lens 5 and the liquid crystal lens 4 functioning as a concave lens again, and The light is returned to linearly polarized light by the plate 3, is separated by the beam splitter 2, and reaches the photodetector 8.

In the above example, the substrate 12 has a concave portion, but if a substrate having the same configuration and having a convex portion is used,
It will function as a convex lens. Further, by using the refractive index of the substrate 12 that is to coincide with the ordinary refractive index n _o of the liquid crystal acts as a convex lens if the substrate 12 is a substrate having a concave portion at the time of voltage off, the convex portion When a substrate having the above is used, it functions as a concave lens.

In this case, in the alignment treatment, the substrates on both sides are subjected to horizontal orientation treatment, only one substrate is subjected to horizontal orientation treatment,
Orientation processing such as horizontal alignment processing of only one substrate and vertical alignment processing of the other substrate, and vertical alignment processing of both substrates can be performed.

In the vertical alignment treatment, an organic silane, lecithin,
It may be performed by a method of treating the surface of the electrode substrate with a surfactant or the like. The horizontal alignment treatment may be performed by a method of rubbing an electrode, a substrate, or an organic or inorganic overcoat material formed on the electrode or substrate with a cloth or the like in one direction, an oblique evaporation method, or the like.

Incidentally, as the light source 1 used in the present invention, a light source used in a usual optical head device can be used. Specifically, a light source using a semiconductor laser is most common, but a light source obtained by combining another laser or a wavelength conversion element can also be used.

The beam splitter 2 diffracts only light having a specific polarization direction. The light from the light source on the outward path passes through as it is, and the light on the return path is diffracted or reflected. Anything that can be reached can be used. Specifically, a diffraction grating, a diffraction grating using liquid crystal, a compound prism, or the like can be used. In particular, a diffraction grating using a liquid crystal that diffracts only light having a specific polarization direction is preferable. The phase difference plate 3
A known retardation plate such as a λ / 4 plate that converts light incident as linearly polarized light into circularly polarized light can be used.

The condenser lens 5 is a lens for condensing light on either the first optical recording medium or the second optical recording medium. When the liquid crystal lens 4 functions as a lens to some extent in both the voltage on state and the off state, the first optical recording medium or the second optical recording medium may be used in any of the use states.
Light can be focused on any of the optical recording media.

[0041]

【Example】

"Example 1" As shown in FIG. 2, a glass substrate having a thickness of 0.5 mm, a size of 10 × 10 mm, and a refractive index of 1.57 was used as the substrates 11 and 12, and the center of the lower glass substrate was The recess 13 was formed in the shape of an aspherical concave lens by pressing. This aspheric lens has a diameter of 2 mm and a center depth of 5 mm.
μm. Both the upper and lower substrates 11 and 12 have electrodes 1
After forming ITO electrodes as Nos. 4 and 15, a polyimide film was applied and rubbed to perform horizontal alignment treatment.

The two substrates 11 and 12 are opposed to each other so that their alignment directions are parallel to each other, and sealed at the periphery to form an empty cell having a gap of 10 μm at the center of the lens and a gap of 5 μm at the periphery. did. Note that antireflection films were formed on the outer surfaces of the substrates 11 and 12, respectively.

In this empty cell, the liquid crystal 17 has an ordinary light refractive index of 1.52, Δn of 0.1, and a twist pitch P of 10 μm.
m of a positive dielectric anisotropic nematic liquid crystal composition,
The inlet was sealed to produce a liquid crystal lens.

As shown in FIG. 1, the transmittance of right-handed and left-handed circularly polarized light having a wavelength of 650 nm was measured by disposing the liquid crystal lens 4, and found to be right-handed circularly polarized light (outgoing path in an optical head device). ), The efficiency was 95% for the left-handed circularly polarized light (return path in the optical head device), and 90% was obtained for the round trip.

First, the substrates 11, 1 above and below the liquid crystal lens 4 are arranged.
A case where no voltage is applied between the two electrodes 14 and 15 will be described. The P-polarized light (polarization direction parallel to the paper) emitted from the light source 1 passes through a polarizing beam splitter 2, and the clockwise circularly polarized light from the phase difference plate 3 is converted into a liquid crystal lens 4.
The light passed through the first optical recording medium 6 without being substantially refracted.

The light reflected by the first optical recording medium 6 becomes left-handed circularly polarized light, passes through the liquid crystal lens 4 again as it is, is returned to linearly polarized light by the phase difference plate 3, and becomes S-polarized light (on the paper surface). It becomes light having a (vertical polarization direction) and is incident on the polarizing beam splitter 2. S-polarized light beam splitter 2
And reached the photodetector 8.

On the other hand, the substrates 11 and 1 above and below the liquid crystal lens 4
A case where a voltage of 100 Hz and 5 V is applied between the two electrodes 14 and 15 will be described. The P-polarized light (polarization direction parallel to the paper surface) emitted from the light source 1 passes through a polarizing beam splitter 2, and the clockwise circularly polarized light from the phase difference plate 3 is refracted by a liquid crystal lens 4. Thus, the second optical recording medium 7 was focused.

The light reflected by the second optical recording medium 7 becomes counterclockwise circularly polarized light, refracted by the liquid crystal lens 4 again, returned to linearly polarized light by the phase difference plate 3, and converted into S-polarized light (perpendicular to the paper surface). (Polarization direction), and enters the polarization beam splitter 2. The S-polarized light is diffracted by the beam splitter 2 and reaches the photodetector 8.

Example 2 Instead of the liquid crystal lens of Example 1, the same glass substrate was used, and as shown in FIG. 3, an uneven portion 23 having a Fresnel lens structure was formed by pressing. The uneven portion 23 of the Fresnel lens structure had a diameter of 2 mm and a center depth of 2 μm. Empty cells were formed in the same manner as in Example 1 except that the gap at the peripheral portion was 4 μm.

In this empty cell, the liquid crystal 27 has an ordinary light refractive index of 1.52, Δn of 0.1, and a twist pitch P of 4 μm.
Was injected, and the injection port was sealed to produce a liquid crystal lens.

This liquid crystal lens was incorporated in an optical head device having a configuration as shown in FIG. When measuring the transmittance of clockwise and counterclockwise circularly polarized light with a wavelength of 650 nm,
95% for clockwise circularly polarized light (outbound path in optical head device),
Efficiency of 95% was obtained even for counterclockwise circularly polarized light (return path in the optical head device), and 90% efficiency was obtained for reciprocation. As in Example 1, the focus could be switched by turning on and off a voltage of 100 Hz and 5 V.

"Example 3" As shown in FIG.
2, the thickness is 0.5 mm and the size is 10 × 10 mm
A glass substrate having a refractive index of 1.62 is used, and the center of the lower glass substrate is pressed into an aspheric concave lens shape by pressing.
3 was formed. This aspheric lens had a diameter of 2 mm and a center depth of 5 μm. ITO electrodes were formed as electrodes 14 and 15 on both the upper and lower substrates 11 and 12. Next, a polyimide film was applied to the substrate 11 on the upper surface, and rubbing was performed to perform a horizontal alignment process. The lower substrate 12
Was coated with an organic silane-based vertical alignment agent.

The two substrates 11 and 12 are opposed to each other so that their alignment directions are parallel to each other, and are sealed at the periphery to form an empty cell having a gap of 10 μm at the center of the lens and a gap of 5 μm at the periphery. did. Note that antireflection films were formed on the outer surfaces of the substrates 11 and 12, respectively.

In this empty cell, the liquid crystal 17 has an ordinary light refractive index of 1.52, Δn of 0.2, and a twist pitch P of 2 μm.
Was injected, and the injection port was sealed to produce a liquid crystal lens.

This liquid crystal lens was incorporated in an optical head device having a configuration as shown in FIG. When measuring the transmittance of clockwise and counterclockwise circularly polarized light with a wavelength of 650 nm,
95% for clockwise circularly polarized light (outbound path in optical head device),
Efficiency of 95% was obtained even for counterclockwise circularly polarized light (return path in the optical head device), and 90% efficiency was obtained for reciprocation. As in Example 1, the focus could be switched by turning on and off a voltage of 100 Hz and 5 V.

[Example 4] 0.5 mm thick as two substrates
A glass substrate having a size of 10 × 10 mm and a refractive index of 1.57 was used. On the lower glass substrate, a convex portion was formed in an aspheric convex lens shape by etching so that the center became a convex portion. This aspheric lens had a diameter of 1.5 mm and a center height of 4 μm. After forming ITO electrodes as electrodes on both the upper and lower substrates, a polyimide film was applied and rubbed to perform horizontal alignment processing.

The two substrates were opposed to each other so that their orientation directions became parallel, and sealed at the periphery to form an empty cell having a gap of 4 μm at the center of the lens and a gap of 8 μm at the periphery. Note that antireflection films were formed on the outer surfaces of the two substrates, respectively.

In this empty cell, the ordinary refractive index of the liquid crystal is 1.49, Δn is 0.12, and the twist pitch P is 3 μm.
Was injected, and the injection port was sealed to produce a liquid crystal lens.

In the liquid crystal lens thus manufactured, when no voltage is applied, the liquid crystal molecules have a helical structure, and the helical axis is perpendicular to the substrate surface. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal is 1.55, which is an intermediate value between the ordinary refractive index of 1.49 and the extraordinary refractive index of 1.61. From this,
The difference between the refractive index of the liquid crystal and the refractive index of the substrate became smaller, and the phase distribution of the light transmitted through the liquid crystal lens was almost the same as before transmission.

Next, when a voltage of 100 Hz and 10 V is applied between the electrodes of the substrate above and below the liquid crystal lens, the liquid crystal molecules are in a vertical alignment state. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal becomes equal to the ordinary light refractive index of 1.49. From this, the difference between the refractive index of the liquid crystal and the refractive index of the substrate increases, and the phase distribution of the light transmitted through the liquid crystal lens is proportional to the height of the aspherical shape formed on the substrate surface of the cell. changed.

Thus, when the voltage is not applied between the electrodes on the upper and lower substrates of the liquid crystal lens 4 when used in the configuration of FIG. 1, the signal from the first optical recording medium 6 can be read and the voltage can be reduced. When the voltage was applied, the signal from the second optical recording medium 7 could be read.

Example 5 The same glass substrate as in Example 4 was used.
However, the glass substrate on the lower surface was formed into a convex convex lens shape by etching so that the center became a convex portion. This aspheric lens has a diameter of 1.5 mm and a center height of 3 μm.
m. On the glass substrate on this lower surface, IT
After forming the O electrode, a vertical alignment treatment was performed by applying an organic silane-based solvent. On the other hand, on the glass substrate on the top, after forming an ITO electrode as an electrode, a polyimide film is applied,
Rubbing was performed to perform horizontal alignment processing.

The two substrates are opposed to each other, sealed at the periphery, and the gap is 3 μm at the center of the lens and 6 mm at the periphery.
A μm empty cell was formed. Note that antireflection films were formed on the outer surfaces of the two substrates, respectively.

In this empty cell, as a liquid crystal, the ordinary light refractive index is 1.49, Δn is 0.12, and the twist pitch P is 1.6.
A nematic liquid crystal composition having a positive dielectric anisotropy of μm was injected, and the injection port was sealed to produce a liquid crystal lens.

In the liquid crystal lens thus manufactured, when no voltage is applied, the liquid crystal molecules have a helical structure, and the helical axis is perpendicular to the substrate surface. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal is 1.55, which is an intermediate value between the ordinary refractive index of 1.49 and the extraordinary refractive index of 1.61. From this,
The difference between the refractive index of the liquid crystal and the refractive index of the substrate became smaller, and the phase distribution of the light transmitted through the liquid crystal lens was almost the same as before transmission.

Next, when a voltage of 100 Hz and 10 V is applied between the electrodes of the substrate above and below the liquid crystal lens, the liquid crystal molecules are in a vertical alignment state. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal becomes equal to the ordinary light refractive index of 1.49. From this, the difference between the refractive index of the liquid crystal and the refractive index of the substrate increases, and the phase distribution of the light transmitted through the liquid crystal lens is proportional to the height of the aspherical shape formed on the substrate surface of the cell. changed.

Thus, when the voltage is not applied between the electrodes on the upper and lower substrates of the liquid crystal lens 4 in the configuration shown in FIG. 1, a signal from the first optical recording medium 6 can be read, and the voltage can be reduced. When the voltage was applied, the signal from the second optical recording medium 7 could be read.

Example 6 As shown in FIG. 4, a glass substrate having a thickness of 0.5 mm, a size of 10 × 10 mm, a refractive index of 1.49 was used as two substrates, and a glass substrate on the lower surface was used. In this method, a concave portion was formed in an aspherical concentric shape by pressing so that the central portion became a peak having the same height as the flat portion of the substrate. This aspheric lens has a diameter of 2.3 mm and a depth of the concave portion of 2.3.
μm. After forming ITO electrodes as electrodes on both the upper and lower substrates, a polyimide film was applied and rubbed to perform horizontal alignment processing.

The two substrates were opposed to each other so that their alignment directions became parallel, and the periphery was sealed to form an empty cell having a gap of 4 μm at the center of the lens and a gap of 6.3 μm at the concave portion. Note that antireflection films were formed on the outer surfaces of the two substrates, respectively.

In this empty cell, as a liquid crystal, the ordinary light refractive index is 1.49, Δn is 0.12, and the twist pitch P is 1.6.
A nematic liquid crystal composition having a positive dielectric anisotropy of μm was injected, and the injection port was sealed to produce a liquid crystal lens.

In the liquid crystal lens manufactured as described above, when no voltage is applied, the liquid crystal molecules have a helical structure, and the helical axis is perpendicular to the substrate surface. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal is 1.55, which is an intermediate value between the ordinary refractive index of 1.49 and the extraordinary refractive index of 1.61. From this,
The difference between the refractive index of the liquid crystal and the refractive index of the substrate increased, and the phase distribution of the light transmitted through the liquid crystal lens changed so as to be proportional to the height of the aspherical shape formed on the substrate surface of the cell.

Next, when a voltage of 100 Hz and 10 V is applied between the electrodes on the upper and lower substrates of the liquid crystal lens, the liquid crystal molecules are in a vertical alignment state. Therefore, for light having a wavelength of 633 nm, which is perpendicularly incident on the substrate surface, the effective refractive index of the liquid crystal becomes equal to the ordinary light refractive index of 1.49. From this, the difference between the refractive index of the liquid crystal and the refractive index of the substrate became small, and the phase distribution of the light transmitted through the liquid crystal lens was almost the same as the state before transmission.

Thus, when the voltage is not applied between the electrodes on the upper and lower substrates of the liquid crystal lens 4 when used in the configuration of FIG. 1, the signal from the second optical recording medium 7 can be read and the voltage can be reduced. When applied, the signal from the first optical recording medium 6 could be read.

[0074]

According to the optical head device of the present invention, since the liquid crystal lens in which the liquid crystal is twisted is used, the focal length or the phase distribution of light can be switched by applying a voltage from the outside, and the optical head with high utilization efficiency can be used. A device can be obtained. The present invention can be applied to various applications as long as the effects are not impaired.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of an optical head device according to the present invention.

FIG. 2 is a cross-sectional view illustrating an example of a liquid crystal lens in which a substrate has a concave portion or a convex portion.

FIG. 3 is a cross-sectional view illustrating an example of a liquid crystal lens in which a substrate has a Fresnel lens structure.

FIG. 4 is a cross-sectional view showing an example of a liquid crystal lens having a mountain at the center of a substrate.

Claims (6)

[Claims] In an optical head device having a light source, a beam splitter, a liquid crystal lens, and a photodetector, as a liquid crystal lens disposed between the beam splitter and an optical recording medium,
At least one of the substrates of the liquid crystal cell has a concave portion or a convex portion, the liquid crystal filled therein is twisted, and the focal length or the phase distribution of light is varied by electrodes provided on at least a part of the substrate. An optical head device characterized by using: 2. An optical head device having a light source, a beam splitter, a liquid crystal lens, and a photodetector, wherein the liquid crystal lens disposed between the beam splitter and the optical recording medium is:
At least one of the substrates of the liquid crystal cell has a Fresnel lens structure having fine concaves or convexes, and the liquid crystal filled therein is twisted and focused by electrodes provided on at least a part of the substrate. An optical head device using a variable distance or phase distribution of light. 3. The optical head device according to claim 1, wherein the substrate at the center of the liquid crystal lens is substantially flat. 4. A liquid crystal ordinary refractive index n _o, the extraordinary refractive index of n _e, when the twisting pitch P, and wavelength in vacuum was _{λ, (n e -n o)} 2 P / (8λ) 4. The optical head device according to claim 1, wherein ≤0.05. 5. The optical head device according to claim 1, wherein the refractive index of the substrate is substantially equal to the ordinary refractive index of the liquid crystal, the extraordinary refractive index, or the average of the ordinary refractive index and the extraordinary refractive index. . 6. The polarization diffraction element according to claim 1, wherein a substrate provided with a lattice-shaped concave portion is used as at least one substrate, and a polarization diffraction element in which an optically anisotropic material is filled between the substrates is used. Optical head device.