JP4501611B2 - Liquid crystal lens element and optical head device - Google Patents

Description

  The present invention relates to a liquid crystal lens element and an optical head device, and in particular, a liquid crystal lens that can be switched to a different focal length depending on the magnitude of an applied voltage, and information recording and recording on an optical recording medium equipped with the liquid crystal lens The present invention relates to an optical head device used for reproduction.

As an optical recording medium (hereinafter referred to as “optical disk”) having an information recording layer formed on the light incident side surface and a cover layer made of a transparent resin covering the information recording layer, DVD and the like have been widely used. Yes. On the other hand, an optical head device used for recording and / or reproducing information on a DVD includes a semiconductor laser having a wavelength of 660 nm as a light source and an objective lens having an NA (numerical aperture) of 0.6 to 0.65. What you have is known.
By the way, a conventionally used DVD has a single information recording layer and a cover thickness of 0.6 mm (hereinafter referred to as “single-layer optical disk”). However, in recent years, in order to increase the amount of information per optical disc, an optical disc (hereinafter referred to as “double-layer optical disc”) having two information recording layers (reproduction-only or reproducible and recordable) has been developed. ing.

When recording and / or reproducing on a two-layer optical disk using an optical head device having an objective lens that is optimally designed to have zero aberration for such a single-layer optical disk, if the cover thickness is different, Spherical aberration occurs according to the difference in the cover thickness, and the condensing property of incident light on the information recording layer is deteriorated. In particular, in a recording type two-layer optical disc, deterioration of light condensing performance is a problem because it leads to a decrease in light condensing power density during recording and causes a writing error.
In recent years, an optical disc having a cover thickness of 0.1 mm (hereinafter referred to as a “high density optical disc”) has been proposed in order to further improve the recording density of the optical disc. On the other hand, an optical head device for recording information on an optical disk has been developed that includes a semiconductor laser that emits blue laser light having a wavelength of 405 nm as a light source and an objective lens having an NA of 0.85. ing. However, even in the case of this type of optical head device, there is a problem with the recording type double-layer optical disk because the spherical aberration that occurs according to the difference in the cover thickness causes a writing error.

Therefore, conventionally, a method using a movable lens group or a liquid crystal lens is known as means for correcting spherical aberration caused by the difference in the cover thickness of the two-layer optical disk as described above.
(I) For example, in order to perform spherical aberration correction using a movable lens group, an optical head device 100 for recording / reproducing an optical disc D as shown in FIG. 16 has been proposed (see, for example, Patent Document 1). ).
In addition to the light source 110, various optical systems 120, the light receiving element 130, the control circuit 140, and the modulation / demodulation circuit 150, the optical head device 100 includes first and second movable lens groups 160 and 170. And. The first movable lens group 160 includes a concave lens 161, a convex lens 162, and an actuator 163. By moving the convex lens 162 fixed to the actuator 163 in the optical axis direction, the first movable lens group 160 includes It exhibits a variable focal length lens function whose power continuously changes from positive (convex lens) to negative (concave lens).
The movable lens group 160 is arranged in the optical path of the optical disc D, thereby correcting spherical aberration including a power component that allows the incident light to be focused on an information recording layer (not shown) having a different cover thickness of the optical disc D. Is possible.
However, when the movable lens group 160 is used, the pair of lenses 161 and 162 and the actuator 163 are required, so that the size of the optical head device 100 is increased and the mechanism design for moving is complicated. was there.

(II) Also, an optical head device using a liquid crystal lens 200 as shown in FIG. 17 has been proposed in order to correct the spherical aberration caused by the difference in the cover thickness of the optical disc (for example, Patent Document 1). 2).
The liquid crystal lens 200 includes a substrate 230 on which a transparent electrode 210 and an alignment film 220 are formed on a flat surface, and an axially symmetrical sum of powers of a radius r.
S (r) = α ₁ r ² + α ₂ r ⁴ + α ₃ r ⁶ +... + Α _m r ^2m +.
(1) where r ² = x ² + y ²
α ₁ , α ₂ , α ₃ ,..., α _m ; sandwiched by a substrate 260 in which a transparent electrode 240 and an alignment film 250 are formed on a curved surface having a surface shape S (r) described by constants. The nematic liquid crystal 270 is provided.
In this liquid crystal lens 200, when a voltage is applied between the transparent electrodes 210 and 240, the molecular orientation of the liquid crystal 270 changes and the refractive index changes. As a result, the wavefront of the transmitted light changes according to the refractive index difference between the substrate 260 and the liquid crystal 270.
Here, the refractive index of the substrate 260 is equal to that of the liquid crystal 270 when no voltage is applied. Therefore, when this voltage is not applied, the transmitted wavefront of the incident light does not change. On the other hand, when a voltage is applied between the transparent electrodes 210 and 240, a refractive index difference Δn is generated between the substrate 260 and the liquid crystal 270, and Δn × S (r) (where S (r) refers to the equation (1)). A phase difference of transmitted light corresponding to () occurs.
Accordingly, the surface shape S (r) of the substrate 260 is processed so as to correct the spherical aberration caused by the difference in the cover thickness of the optical disc D, and the refractive index difference Δn is adjusted according to the applied voltage. Aberration correction is possible.
However, in the case of the liquid crystal lens shown in FIG. 17, since the refractive index change of the liquid crystal 270 with respect to the applied voltage is about 0.3 at the maximum, a large phase difference distribution Δn × corresponding to the power component that changes the focus of incident light. In order to generate S (r), the unevenness difference of S (r) must be increased. As a result, the layer of the liquid crystal 270 becomes thick, causing a problem that the drive voltage increases and the response becomes slow.
Therefore, in order to make the liquid crystal layer thin, it is effective to correct only the spherical aberration with the smallest aberration correction amount excluding the power component. However, when the surface shape S (r) of the substrate 260 is processed so as to correct only the spherical aberration, the optical axis of the objective lens that collects incident light on the information recording layer of the optical disc and the optical axis of the liquid crystal lens are decentered. Then, coma aberration occurs, and the light condensing property to the information recording layer is deteriorated, which causes a problem that recording and reproduction cannot be performed.

(III) A liquid crystal diffractive lens 300 as shown in FIG. 18 has also been proposed in order to develop a substantial lens function that can change the power component corresponding to the focus change of incident light without increasing the thickness of the liquid crystal layer. (For example, refer to Patent Document 3).
In this liquid crystal diffractive lens 300, a transparent electrode 320 is formed on one surface of a substrate 310 on which a predetermined serrated relief is formed, and the liquid crystal layer 340 is sandwiched between the transparent electrode 320 and the counter electrode 330. When a voltage is applied between the electrodes 320 and 330, substantial refractive index of the liquid crystal layer 340 with respect to extraordinarily polarized light changes from extraordinary refractive index n _e to the ordinary refractive index n _o. Here, the substantial refractive index means the average refractive index in the thickness direction of the liquid crystal layer.
When the refractive index of the substrate 310 having a serrated relief structure is n ₁ and the wavelength of incident light is λ, the depth d of the groove of the serrated relief satisfies the relationship of the following equation:
_{d = λ / (n e -n} 1)
By forming, a maximum diffraction efficiency is obtained at a wavelength λ when no voltage is applied, and a diffractive lens is obtained. Further, even if the wavelength λ of the incident light changes, the applied voltage can be adjusted so that the maximum diffraction occurs at the wavelength λ.
In the liquid crystal diffractive lens 300 having such a configuration, it is only necessary to fill the liquid crystal layer 340 with liquid crystal so as to fill the groove of the serrated relief, so that the spherical aberration including the power component using the liquid crystal lens 200 shown in FIG. The liquid crystal layer 340 can be made thinner than a liquid crystal layer that corrects the above.
However, in the liquid crystal diffractive lens 300, the refractive index difference between the sawtooth relief structure and the liquid crystal changes with the applied voltage, and the product of the refractive index difference and the groove depth d of the sawtooth relief is an integral multiple of the wavelength λ. Only in such a state, the wavefronts are continuously connected and a power component can be obtained, so that the generated power can be changed only discretely according to the voltage.
On the other hand, the cover thickness of the optical disc generally varies due to manufacturing errors. In particular, in an optical head device including an objective lens having an NA of 0.85, spherical aberration caused by variations in the cover thickness is proportional to the fourth power of the objective lens NA, which causes a manufacturing error in the cover thickness. It is necessary to appropriately correct spherical aberration. Therefore, in the liquid crystal diffractive lens 300 in which only discrete power components can be obtained, it is difficult to properly correct the manufacturing variation of the cover thickness.

Further, in the liquid crystal lens shown in FIG. 17 and the liquid crystal diffractive lens as shown in FIG. 18, since the liquid crystal is uniformly oriented, the transmitted wavefront is applied at an applied voltage for polarized light that senses the ordinary refractive index of the liquid crystal. It cannot be changed. Since an optical head device used for recording and reproduction of a DVD or a high-density optical disk generally uses a polarization optical system, forward light (light toward the optical disk) having orthogonal polarization and return light (reflected from the optical disk) There arises a problem that only one of the light) can correct the spherical aberration.
JP 2003-115127 A Japanese Patent Laid-Open No. 5-205282 JP-A-9-189892

  The present invention has been made in view of the above circumstances, and it is possible to realize a small element having no movable part, and it is possible to realize stable incident light according to the magnitude of an applied voltage while the liquid crystal layer is a thin liquid crystal element. An object of the present invention is to provide a liquid crystal lens element having a lens function capable of correcting spherical aberration including a power component corresponding to a focus change. In addition, the present invention corrects spherical aberration caused by a difference in cover thickness between a single-layer optical disk and a double-layer optical disk by using this liquid crystal lens element, and can perform stable recording and / or reproduction. The purpose is to provide.

  The present invention provides a liquid crystal lens element that changes a focal length of light transmitted through the liquid crystal according to a magnitude of a voltage applied to the liquid crystal, and includes a first Fresnel lens unit, a second Fresnel lens unit, The first Fresnel lens unit is disposed on the surface of the transparent substrate for applying a voltage to the first liquid crystal layer sandwiched between the pair of transparent substrates. A transparent material on at least one upper surface of the opposing electrode pair, and a sawtooth-like cross-sectional shape having rotational symmetry with respect to the optical axis of the light or a cross-sectional shape approximating a sawtooth to a step shape The second Fresnel lens portion includes a second liquid crystal layer sandwiched between a pair of transparent substrates, and a surface of the transparent substrate for applying a voltage to the second liquid crystal layer. In each A transparent material on at least one upper surface of the opposing electrode pair, and a sawtooth-like cross-sectional shape having rotational symmetry with respect to the optical axis of the light or a cross-sectional shape approximating a sawtooth to a step shape The electrode lens portion is opposed to the surface of the transparent substrate for applying a voltage to the third liquid crystal layer, and a third liquid crystal layer sandwiched between a pair of transparent substrates. At least one of which is a composite electrode composed of a low resistance electrode and a high resistance planar electrode, and the first, second and third liquid crystal layers are not applied with voltage or applied with voltage. 2. A liquid crystal lens element, wherein the liquid crystal lens element is a nematic liquid crystal aligned in parallel, wherein an ordinary light refractive index direction of the first liquid crystal layer and an extraordinary light refractive index direction of the second liquid crystal layer and the third liquid crystal layer coincide. I will provide a.

  In addition, any one of the opposing electrode pairs installed in the electrode lens unit is the composite electrode, and the composite electrode includes a high-resistance planar electrode and a plurality of concentric circles centered on the optical axis of the light. A liquid crystal lens element comprising the low-resistance electrode is provided.

Further, both of the opposing electrodes installed in the electrode lens unit are the composite electrodes,
The one composite electrode includes a high resistance planar electrode and a plurality of low resistance electrodes arranged in a stripe pattern, and the other composite electrode includes an arrangement direction of the high resistance planar electrode and the low resistance electrode. Provided is a liquid crystal lens element comprising a plurality of low resistance electrodes arranged in stripes in a direction perpendicular to each other.

  In addition, the liquid crystal lens element is characterized in that a refractive index of the transparent material forming the first and second concavo-convex portions is equal to an ordinary refractive index of the first liquid crystal layer and the second liquid crystal layer. .

  The first Fresnel lens portion, the second Fresnel lens portion, and the electrode lens portion are laminated and integrated, and the first, second, and third liquid crystal layers include four sheets facing each other. The liquid crystal lens element is provided, wherein the liquid crystal lens element is provided in a gap between three substrates formed by a transparent substrate.

  The liquid crystal lens element is characterized in that a phase plate whose phase difference with respect to the wavelength of the light is an odd multiple of π / 2 is integrated.

The electrode lens portion of the liquid crystal lens element includes a first electrode lens portion and a second electrode lens portion,
The first electrode lens unit is disposed on the surface of the transparent substrate in order to apply a voltage to the third liquid crystal layer sandwiched between a pair of transparent substrates, and at least one of them. An opposing electrode pair that is a composite electrode composed of a low-resistance electrode and a high-resistance planar electrode;
The second electrode lens unit is disposed on the surface of the transparent substrate in order to apply a voltage to the fourth liquid crystal layer sandwiched between the pair of transparent substrates, and at least one of them is low An opposing electrode pair that is a composite electrode composed of a resistance electrode and a high-resistance planar electrode;
The first, second, third, and fourth liquid crystal layers are nematic liquid crystals that are aligned in parallel when no voltage is applied or when a voltage is applied, and the normal refractive index direction of the first liquid crystal layer and the second and third liquid crystals. The liquid crystal lens element is provided in which the extraordinary refractive index direction of the layer and the ordinary refractive index direction of the fourth liquid crystal layer coincide with each other.

One of the opposing electrodes installed in the electrode lens part is the composite electrode,
This composite electrode is provided with the liquid crystal lens element described above, characterized by comprising a high resistance flat electrode and a plurality of concentric low resistance electrodes centered on the optical axis of the light.

Further, the opposing electrodes installed in the electrode lens part are both the composite electrodes,
One of the composite electrodes is composed of a high resistance flat electrode and a plurality of low resistance electrodes arranged in a stripe pattern,
The other composite electrode is composed of a plurality of low resistance electrodes arranged in stripes in a direction orthogonal to the arrangement direction of the high resistance planar electrode and the low resistance electrode. I will provide a.

  In addition, the liquid crystal lens element is characterized in that a refractive index of the transparent material forming the first and second concavo-convex portions is equal to an ordinary refractive index of the first liquid crystal layer and the second liquid crystal layer. .

  In addition, the first Fresnel lens part and the second Fresnel lens part are laminated and integrated, and the first electrode lens part and the second electrode lens part are laminated and integrated. A liquid crystal lens element according to claim 7 to 10 is provided.

  The present invention also provides a light source, an objective lens for condensing the light emitted from the light source on the optical recording medium, and a photodetector for detecting the light collected and reflected on the optical recording medium. An optical head device comprising: the liquid crystal lens element according to claim 1 disposed in an optical path between the light source and the objective lens.

  Further, on the optical path of light traveling from the light source to the optical recording medium, the polarization direction of the light from the light source incident on the liquid crystal lens element is an extraordinary refractive index direction of the third liquid crystal layer of the liquid crystal lens element. The above-described optical head device is provided that matches.

  According to another aspect of the present invention, there is provided a light source, an objective lens for condensing light emitted from the light source on the optical recording medium, and an optical path between the light source and the objective lens. An optical head device comprising the liquid crystal lens element according to the item is provided.

The present invention also provides a light source, an objective lens that focuses light emitted from the light source onto the information recording layer of the optical recording medium, a photodetector that receives the reflected light of the information recording layer, and an optical recording medium from the light source. In an optical head device including at least a beam splitter that separates a light beam on an outward path toward the light beam and a light beam on a return path where reflected light from the information recording layer faces a photodetector,
A forward liquid crystal lens element in which the first Fresnel lens part and the second electrode lens part are laminated and integrated, and a backward liquid crystal in which the second Fresnel lens part and the first electrode lens part are laminated and integrated. 11. The liquid crystal lens element according to claim 7, wherein the forward liquid crystal lens element is disposed in an optical path between the light source and the beam splitter, and the backward liquid crystal lens element is disposed between the beam splitter and the photodetector. Provided is an optical head device which is arranged in an optical path.

According to the present invention, since the transmitted wavefront changes according to the applied voltage, a variable focal length liquid crystal lens can be realized. In addition, in the first and second Fresnel lens portions provided in the liquid crystal lens element of the present invention, the liquid crystal is filled in the concave portions of the transparent material having a sawtooth shape or a cross-sectional shape approximating the sawtooth in a stepped shape. Although a large power component can be generated, the thickness of the liquid crystal layer can be reduced, leading to low voltage driving and high speed response. Furthermore, since the extraordinary refractive index directions of the first and second liquid crystal layers are orthogonal, stable spherical aberration correction can be realized regardless of the incident polarization. Further, according to the present invention, the power component can be continuously changed by the electrode lens portion provided in the liquid crystal lens element.
Therefore, in the optical head device provided with such a liquid crystal lens element, not only the spherical aberration generated due to the difference in the cover thickness in the two-layer optical disc but also the spherical surface generated due to the variation in the cover thickness. Aberrations can also be corrected effectively. Also, even when the objective lens is decentered from the liquid crystal lens element during tracking, there is little aberration deterioration, so that an optical head device capable of stable recording and / or reproduction can be provided.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[First Embodiment]
A configuration example of the liquid crystal lens element according to the first embodiment of the present invention will be described below.
FIG. 1 is a cross-sectional view showing a first embodiment of the liquid crystal lens element of the present invention. The liquid crystal lens element 10 according to the present embodiment includes a first Fresnel lens unit 10A, a second Fresnel lens unit 10B, and an electrode lens unit 10C as a general configuration, and includes four transparent substrates 11 to 11. 14 and three liquid crystal layers 24 to 26 are laminated and integrated.

FIG. 2 is a top view showing the Fresnel lens portion 10A (or 10B) of the liquid crystal lens element according to the first embodiment of the present invention. FIG. 3 is a top view of the electrode lens unit 10C according to the first embodiment of the liquid crystal lens element of the present invention.
The first Fresnel lens portion 10 </ b> A includes the transparent substrates 11 and 12, the first liquid crystal layer 24 sandwiched between the transparent substrates 11 and 12 and the seal 21, the first uneven portion 27, and the first liquid crystal layer 24. Are provided with opposing transparent electrodes 15 and 16 for applying a voltage thereto. Similarly, the second Fresnel lens unit 10B includes the transparent substrates 12 and 13, the second liquid crystal layer 25 sandwiched between the transparent substrates 12 and 13 and the seal 22, the second uneven portion 28, and the second Opposing transparent electrodes 17 and 18 for applying a voltage to the liquid crystal layer 25 are provided.
On the other hand, the electrode lens unit 10 </ b> C includes transparent substrates 13 and 14, a third liquid crystal layer 26 sandwiched between the transparent substrates 13 and 14 and the seal 23, and a transparent electrode for applying a voltage to the third liquid crystal layer 26. 19 and a composite electrode 20 are provided.
The transparent electrodes 15, 17 and 16, 18 are connected to the external signal source 30 by electrode extraction parts 15 A, 17 A and 16 A, 18 A and conduction connection means 29. The transparent electrode 19 is connected to the external signal source 30 via the electrode extraction part 19A and a connection line (not shown). As shown in FIG. 3, the composite electrode 20 includes low resistance electrodes 31 to 34 arranged concentrically around the optical axis, and a uniform high resistance flat electrode 35. Among these, the low resistance electrodes 31 to 34 are connected to the external signal source 30 via the electrode extraction portions 31A to 34A and connection wires (not shown).
The concavo-convex portions 27 and 28 have a cross section having a sawtooth shape or a shape approximating a sawtooth stepwise, and are formed using a uniform refractive index transparent material. In the region of the effective diameter φ, the optical axis of the incident light It has rotational symmetry with respect to (Z axis). Details of the uneven portions 27 and 28 will be described later.

Next, an example of a manufacturing procedure of the liquid crystal lens element 10 will be described below.
First, transparent electrodes 15 to 19 are formed on one surface of the transparent substrate 11 and both surfaces of the transparent substrates 12 and 13. Further, the upper surface of the transparent electrodes 16 and 18, a uniform refractive index transparent material having a refractive index n _s, cross section form a concave-convex portions 27 and 28 having a shape approximating a sawtooth or sawtooth stepwise. The concavo-convex portions 27 and 28 may be processed into a concavo-convex shape by photolithography or reactive ion etching after forming a uniform refractive index transparent material layer having a predetermined thickness on the surfaces of the transparent electrodes 16 and 18. The shape of the concavo-convex portion may be transferred to the uniform refractive index transparent material layer using a mold. As shown in FIG. 3, low resistance electrodes 31 to 34 are formed on one surface of the transparent substrate 14, and then a high resistance flat electrode 35 is formed to form the composite electrode 20.
Next, the surfaces of the transparent electrode 15 and the concavo-convex portion 27 are subjected to a parallel alignment process so that the extraordinary refractive index direction of the first liquid crystal layer 24 faces the Y direction, and the transparent electrodes 17 and 19, the concavo-convex portion 28, and The surface of the composite electrode 20 is subjected to a parallel alignment treatment so that the extraordinary refractive index direction of the second and third liquid crystal layers 25 and 26 is in the X direction. The alignment process includes spin coating an alignment film composed mainly of polyimide or the like on the substrate surface, followed by rubbing with a cloth or the like, a method of forming an oblique SiO deposition film on the substrate surface, or a photo alignment film on the substrate surface. A method of irradiating polarized ultraviolet rays after spin coating may be used.
Next, an adhesive material (not shown) mixed with a gap control material is printed and patterned to form seals 21 to 23, and the transparent substrates 11 to 14 are overlapped and pressure-bonded to produce an empty cell. A liquid crystal having an ordinary light refractive index n _o and an extraordinary light refractive index n _e (where n _o ≠ n _e ) is injected from an injection port (not shown) provided in a part of the seals 21 to 23. And the liquid crystal is sealed in the cell to obtain the liquid crystal lens element 10 of the present embodiment.

Next, the operation principle in the first embodiment of the liquid crystal lens element of the present invention will be described below.
The liquid crystal lens element 10 of the present invention is composed of first Fresnel lens portions 10A and 10B in which liquid crystal alignment directions are orthogonal, and an electrode lens portion 10C having a liquid crystal alignment direction parallel to the second Fresnel lens portion 10B. Yes. The Fresnel lens portions 10A and 10B change discretely by changing the substantial refractive index of the liquid crystal layers 24 and 25 by changing the voltage applied between the transparent electrodes 15 and 16 or the transparent electrodes 17 and 18. It functions as a Fresnel lens with variable focus.
On the other hand, the electrode lens unit 10C functions as a continuously variable focus lens by changing the substantial refractive index distribution of the liquid crystal layer 26 according to the voltage distribution generated in the composite electrode 20.

Hereinafter, the Fresnel lens portions 10A and 10B and the electrode lens portion 10C will be described in detail in order.
Description of Fresnel Lenses 10A and 10B In order to generate a transmitted wavefront to which a positive or negative power component has been applied using the liquid crystal lens 10 of the present invention, the optical axis of the transmitted wavefront incident on the liquid crystal lens 10 is The phase difference φ of a light beam passing through a position separated by a radius r with respect to the light beam at the center (coordinate origin: x = y = 0) is a power series as shown in the following equation:
φ (r) = a ₁ r ² + a ₂ r ⁴ + a ₃ r ⁶ + a ₄ r ⁸ + (3) where r ² = x ² + y ²
a ₁ , a ₂ ,...;
Here, a specific example of a curve in which the horizontal axis is the radius r and the phase difference φ is expressed in units of the wavelength λ of the incident light is indicated by reference signs P1 and P2 in FIG.
In the case of incident light having a coherent wavelength λ having the same phase, transmitted wavefronts having a phase difference that is an integral multiple of λ can be regarded as equivalent. Therefore, the graphs F1 and F2 showing the phase differences obtained by dividing the graphs indicated by P1 and P2 in FIG. 4 by the wavelength λ interval and moving to the plane of zero phase difference are substantially equivalent to the graphs P1 and P2. The phase difference distributions shown in the graphs F1 and F2 are all within λ and have a sawtooth cross section.
In order to obtain a phase difference corresponding to the graphs F1 and F2 by the liquid crystal lens element 10, the shape of the uneven portions 27 and 28 provided in the Fresnel lens portion 10A or 10B is similar to the shapes of the graphs F1 and F2. Process it. Here, the concavo-convex portions 27 and 28 may be a uniform refractive index transparent material, and may be an organic material such as an ultraviolet curable resin, a heat effect resin, or a photosensitive resin, or may be SiO ₂ , Al ₂ O _3, or SiO _x N. An inorganic material such as _y (where x and y indicate the element ratio of O and N) may be used. Since these materials have an extremely large volume resistivity as compared with the materials constituting the transparent electrodes 15 to 18 and are not sufficiently small as compared with the liquid crystal material, they can be regarded as dielectrics.

FIG. 5 is an enlarged view of the Fresnel lens portion in the cross-sectional view of the liquid crystal lens element of the present invention.
The distance between the transparent electrodes 15 (or 17) and 16 (or 18) formed on the surface of the transparent substrate 11, 12 (or 12, 13) is G, and the film thickness of the uneven portion 27 (or 28). d _F is distributed from zero to d, and the layer thickness d _LC of the liquid crystal layer 24 (or 25) is distributed from G to Gd. Here, the gap G (= d _F + d _LC ) is a constant value.
Since the uneven portion 27 is disposed between the transparent electrodes 15 and 16, an effective voltage V _LC applied to the liquid crystal layer 24 depends on the relative dielectric constant ε _F of the material constituting the uneven portion 27. Change. Specifically, assuming that the alternating voltage V is applied between the electrodes 15 and 16, V _LC / V is described by the following equation.
V _LC / V = 1 / {1+ (ε _LC / ε _F ) × (d _F / d _LC )}
... (4)
Here, the film thickness d _F of the concavo-convex portion 27 is distributed from zero to d corresponding to the sawtooth shape forming the Fresnel lens or the cross-sectional shape approximating the sawtooth in a staircase shape, so d _F / d _LC is from zero to d / (G−d). As a result, the effective voltage V _LC applied to the liquid crystal layer 24 has a spatial distribution according to the shape of the uneven portion 27.
In addition, liquid crystal has dielectric anisotropy, and the relative dielectric constant ε _{// in the} major axis direction of the liquid crystal molecule and the relative dielectric constant ε _{の in the} minor axis direction of the liquid crystal molecule are different. The relative dielectric constant ε _LC of the liquid crystal layer 24 also changes due to the change in the orientation of the liquid crystal molecules. Therefore, in the equation (4), the spatial distribution of the effective voltage V _LC applied to the liquid crystal layer 24 according to the shape of the concavo-convex portion 27 is determined, reflecting the change according to the V _LC of the relative dielectric constant ε _LC. . _{Here, V LC} changes according to the thickness _{d F,} future, the applied voltage is denoted by _V LC _{[d F].} Note that V _LC [0] is equal to the inter-electrode applied voltage V at a position where the film thickness d _F is zero.

By the way, since the voltage V _LC applied to the liquid crystal layer 24 varies depending on the shape of the concavo-convex portion 27, there is a spatial distribution in the substantial refractive index n (V _LC [d _F ]) of the liquid crystal layer 24 with respect to abnormal light polarization. Arise. For example, in FIG. 5, the optical path length between the electrodes 15 and 16 at the position of the film thickness d _F of the uneven portion 27 is n _s × d _F + n (V _LC [d _F ]) × d _LC , and the uneven portion 27 The phase difference φ _{dF with} respect to the optical path length n (V) × G at the center position (d _F = 0) of the Fresnel lens without any of the following expressions is:
φ _dF = {n _s × d _F + n (V _LC [d _F ]) × (G−d _F )}
−n (V) × G (5)
Here, the thickness d _F distributed from zero to d, the phase difference phi _dF is distributed from zero to phi _d follows.
φ _d = {n _s × d + n (V _LC [d]) × (G−d)} − n (V) × G
= {N (V _LC [d])-n (V)} × G
− {N (V _LC [d]) − n _s } × d
For example, at an applied voltage _{V +1,} to generate a phase difference of transmission wavefront corresponding to the graph F1 of FIG. 4, approximately lambda (i.e., 0.75Ramuda～1.25Ramuda) phase difference phi _d is such that , and determines the distance G thickness d and the transparent electrode of the uneven portion may be the thickness of the concave-convex portion 27 is a cross-sectional shape extending from zero to d _F.

Here, by changing the applied voltage V, the phase difference of the equation (5) changes. For example,
i) When the film thickness d _F of the concavo-convex portion 27 is distributed from zero to d, there is an applied voltage V _{0 at} which the phase difference of the equation (5) becomes a sufficiently small value with respect to the wavelength λ of the incident light. At this time, the transmitted wavefront of the liquid crystal lens element 10 does not change. Here, the sufficiently small phase difference is specifically λ / 5 or less, more preferably λ / 10 or less. Also,
ii) At an applied voltage V ₋₁ where the phase difference φ _d is approximately −λ (ie, −0.75λ to −1.25λ), a transmitted wavefront having a phase difference shown in the graph F2 of FIG. 4 can be generated. This corresponds to a transmitted wavefront having a phase difference symmetrical to the graph F1 in FIG. 4 with respect to a plane having no phase difference.
Accordingly, the applied voltage _{V +1,} by switching the V _{0, V -1,} it is possible to switch three transmitting wavefront selectively.
Here, when a plane wave is incident on the liquid crystal lens 10 at the applied voltages V ₊₁ , V ₀ , and V ₋₁ , they are emitted as transmitted wave fronts shown in FIGS. 6A, 6 B, and 6 C, respectively. That is, a lens function corresponding to positive power, no power, and negative power is obtained according to the applied voltage of the transparent electrodes 15 and 16 or the transparent electrodes 17 and 18. Low voltage drive because the degree of freedom in designing the electro-optical characteristics of the phase difference obtained is high by selecting the refractive index and relative permittivity of the liquid crystal and the uneven portion 27, the film thickness d of the uneven portion 27, the transparent electrode interval G, and the like. Alternatively, a wide variety of transmitted wavefronts can be generated.

The above is the case where the light incident on the Fresnel lens is an extraordinary light polarization. In the case of ordinary light polarization, the effective refractive index of the liquid crystal perceived by the incident polarization is always the ordinary refractive index of the liquid crystal, regardless of the applied voltage. Matches. Therefore, in the liquid crystal lens element 10, the phase difference φ _d at the position where the film thickness is d _F with respect to the lowest portion (d _F = 0) of the uneven portion 27 is the ordinary light polarization.
φ _d = (n _s −n _o ) × d (6)
, And the proportion to the difference between the refractive index n _s and the liquid crystal of the ordinary refractive index n _o of the concave-convex portion 27. Here, it is desirable to equalize the refractive index n _s and the liquid crystal of the ordinary refractive index n _o of the concave-convex portion 27. With such a configuration, the ordinary light because phi _d regardless of the magnitude of the applied voltage to the polarization becomes zero, transmission wavefront does not change.
Besides the liquid crystal lens element for generating F1, F2 is the phase difference, separated by wavelength λ intervals the phase difference shown in FIG. 4 of P1, P2, phase difference phi _d is approximately mλ (m = 2 or 3) A corresponding liquid crystal lens element may be used. In this case, the transmission wavefront corresponds to a phase difference obtained by dividing P1 and P2 in FIG. 4 at intervals of wavelength m · λ (here, m = 2 or 3).
In the present embodiment, the element structure and the principle of operation of the liquid crystal lens element 10 that generates the axially symmetric phase difference described by the expression (3) have been described. A liquid crystal lens element that generates a phase difference corresponding to correction of coma, astigmatism, and the like can also be produced by processing the concavo-convex shape of a uniform refractive index transparent material and filling the concave portion with liquid crystal on the same principle.
In addition, when the absolute value of the phase difference to be corrected is equal to or less than the wavelength λ of the incident light, the cross-sectional shape of the uneven portions 27 and 28 made of the uniform refractive index transparent material of the liquid crystal lens element 10 does not need to be a sawtooth shape. Any shape that matches the target wavefront may be used. In this case, the phase difference changes continuously according to the magnitude of the applied voltage.
Further, in the present embodiment, the material forming the concave-convex portion is a uniform refractive index transparent material having a refractive index n _s, the molecular orientation direction of the birefringent material such as a polymer liquid crystal aligned in one direction in the substrate plane It may be used. In this case, the extraordinary refractive index of the birefringent material and n _s, together with the ordinary refractive index equal to the ordinary refractive index n _o of the liquid crystal, the liquid crystal molecules molecular orientation direction of the birefringent material (the direction of the extraordinary refractive index) It is preferable to match the orientation direction. With such a configuration, the normal light refractive index of the liquid crystal and the birefringent material matches the normal light polarized incident light regardless of the magnitude of the applied voltage, so that the transmitted light wavefront does not change.
Moreover, in this embodiment, the thing of the structure which applies an alternating voltage to the liquid crystal layers 24 and 25 via the transparent electrodes 15 and 16 and the transparent electrodes 17 and 18 was shown, respectively. In the present invention, other than this, for example, at least one of the transparent electrodes 15 and 17 and the transparent electrodes 16 and 18 may be divided electrodes that can be spatially divided and applied with different AC voltages independently. Thereby, further various phase difference distributions can be generated.

(2) Description of Electrode Lens Part 10C Next, the electrode lens part 10C provided in the liquid crystal lens element 10 of the present invention will be described below.
The electrode lens unit 10C is intended to give a power component that continuously changes to light having a polarization component that matches the extraordinary refractive index direction of the liquid crystal layer 26. Therefore, as shown in FIG. 3, the composite electrode 20 provided on one side of the electrode lens portion 10C is used to generate a voltage distribution corresponding to the target power component in the high resistance flat electrode 35 and the high resistance flat electrode 35. Low resistance electrodes 31-34.
FIG. 7 is a schematic diagram showing a phase difference distribution generated by the electrode lens unit 10C. When different voltages are applied to the low resistance electrodes 31 to 34, a voltage distribution that continuously changes in accordance with the voltage difference between the low resistance electrodes 31 to 34 is formed on the high resistance planar electrode 35. Since the orientation of the liquid crystal molecules changes according to the voltage distribution, an effective refractive index distribution is formed in the liquid crystal layer 26, and a phase difference distribution is generated.
Each point of A, B, C, and D shown in FIG. 7 corresponds to the position of the low resistance electrodes 31, 32, 33, and 34, and generates a phase difference β that substantially matches the target power α. Here, “substantially match” means that the standard deviation of the difference between the target power α and the phase difference β should be less than 1/20 of the wavelength λ of incident light, and low enough to satisfy this. It is desirable to set the shape of the resistance electrodes 31 to 34 and the voltage to be applied in order to obtain sufficient imaging performance.
The high resistance planar electrode 35 may be a transparent material having a sufficiently high sheet resistance compared to the low resistance electrodes 31 to 34, and is a composition containing an oxide such as zinc, lead, tin, or indium. May be. The low resistance electrodes 31 to 34 may also be transparent compositions containing oxides such as zinc, lead, tin, and indium. If there is no optical problem, a metal film such as aluminum, gold, silver, or chromium is used. It may be.
In the configuration example of FIG. 3, the low resistance electrodes 31 to 34 are connected to the electrode extraction portions 31 </ b> A to 34 </ b> A (not shown) and connected to the external signal source 30. In this case, the voltage applied by the external signal source 30 may be divided and distributed to the low resistance electrodes 31 to 34 by connecting the low resistance electrodes 31 to 34 with an appropriate thin film resistor. This configuration is preferable because the number of signals from the external signal source 30 can be reduced.
In the present embodiment, the operation principle of the electrode lens unit 10C that generates the power component by the composite electrode 20 shown in FIG. 3 has been described. However, in addition to the power component, the structure of the composite electrode 20 can be changed. Axisymmetric spherical aberration can also be generated on the same principle. Furthermore, a phase difference distribution composed of both power and spherical aberration components can be obtained.

As described above, when the liquid crystal lens element of the present invention is used, the generated power can be switched discretely without depending on the polarization, and the polarization matching the liquid crystal alignment direction of the electrode lens portion is continuous. Can be obtained.
In this embodiment, a liquid crystal having a positive dielectric anisotropy that is aligned parallel to the substrate surface when no voltage is applied and liquid crystal molecules are aligned in a direction perpendicular to the substrate surface according to the magnitude of the applied voltage is used. Although an example is shown, another liquid crystal alignment or liquid crystal material may be used. For example, a liquid crystal having negative dielectric anisotropy that is aligned perpendicular to the substrate surface when no voltage is applied and liquid crystal molecules are aligned in a direction parallel to the substrate surface according to the applied voltage V may be used.
Further, the first and second Fresnel lens portions and the electrode lens portion constituting the liquid crystal lens element of the present invention do not need to be integrated as long as they are installed at appropriate positions with respect to the optical axis. However, with such a configuration, there is a problem that the installation space of the liquid crystal lens element becomes large and the position adjustment becomes complicated. Therefore, as shown in FIG. . In this case, four or more substrates may be used to sandwich the three liquid crystal layers. However, it is preferable to stack the four liquid crystal layers because the thickness of the liquid crystal lens element can be reduced.
Further, optical components such as a retardation plate, a diffraction grating, a birefringent hologram element, and a wavelength-dependent diffraction grating may be appropriately laminated and integrated on the surface of the liquid crystal lens element of the present invention. This is preferable because the number of optical components to be configured is reduced and the assembly of the optical head device is simplified. Further, the optical component may be molded on a transparent substrate or may be bonded.

[Second Embodiment]
Next, a liquid crystal lens element according to a second embodiment of the present invention will be described with reference to FIG.
FIG. 8 is a top view of a composite electrode provided in the second embodiment of the liquid crystal lens element of the present invention. The liquid crystal lens element of the present embodiment is the same as the liquid crystal lens element 10 according to the first embodiment shown in FIG. 1 except that the transparent electrode 19 and the composite electrode 20 provided in the electrode lens portion 10C are replaced with the composite electrode shown in FIG. The configuration is replaced with 40 and 50. Accordingly, in the present embodiment, the components other than the electrode lens unit 10C are the same as those in the first embodiment, and thus description thereof will be omitted below, and only the electrode lens unit 10C will be described.

The composite electrodes 40 and 50 provided in the electrode lens portion 10C of the present embodiment are a pair of composite electrodes for applying a voltage to the liquid crystal layer 26, and the low resistance electrodes 41 to 44 and 51 arranged in a stripe shape. A potential distribution is generated in the high-resistance planar electrodes 45 and 55 by applying an appropriate voltage to .about.54.
Here, the phase distribution generated by the electrode lens unit of the present embodiment will be described with reference to FIG. When different voltages are applied to the low resistance electrodes 41 to 44, a potential distribution that continuously changes according to the voltage difference between the low resistance electrodes 41 to 44 is formed on the high resistance flat electrode 45. When the positions of the low resistance electrodes 41 to 44 arranged in a stripe shape correspond to A, B, C, and D shown in FIG. 7, a phase difference β that substantially matches the target power α in the X direction is obtained. A voltage distribution to be generated can be obtained. On the other hand, regarding the Y direction, the voltage distribution generated in the composite electrode 40 does not change in the Y direction. When the positions of the low resistance electrodes 51 to 54 arranged in a stripe shape in the Y direction correspond to A, B, C, and D in FIG. 7, the Y direction substantially matches the target power α. A voltage distribution that generates the phase difference β can be obtained.

Therefore, the effective voltage applied to the liquid crystal layer 26 disposed between the two composite electrodes 40 and 50 that generate a voltage distribution that changes in the X direction and the Y direction is the transparent electrode 19 in the first embodiment. And the distribution is the same as that of the composite electrode 20 (see FIG. 1). Accordingly, an effective refractive index distribution is formed in the liquid crystal layer 26, and a phase difference β substantially matching the target power α can be obtained.
The materials and manufacturing methods of the low resistance electrodes 41 to 44, 51 to 54 and the high resistance flat electrodes 45 and 55 may be the same as those of the composite electrode 20 in the first embodiment. Further, when the low resistance electrodes 41 to 44 or the low resistance electrodes 51 to 54 are connected to the external signal source 30, the low resistance electrodes may be connected by a thin film resistor in order to reduce the number of signal lines. This is preferable because it can be driven by the number of signal lines.

[Third Embodiment]
Next, an optical head device equipped with the liquid crystal lens element of the present invention will be described below.
FIG. 9 is a schematic diagram showing an example of an optical head device 60 on which the liquid crystal lens element of the present invention is mounted. This is an optical head device for recording and / or reproducing information on a two-layer optical disk D. A polarizing beam splitter 62, a collimator lens 63, a liquid crystal lens element 64 according to the present invention, a quarter-wave plate 65, an objective lens 66, a cylindrical lens 67, and a photodetector 68. Yes. As the two-layer optical disc D, a DVD, a high-density optical disc, or the like having a first recording layer D1 and a second recording layer D2 is used.
The wavelength of the semiconductor laser 61 may be any one of the 780 nm band, the 660 nm band, and the 405 nm band depending on the type of the optical disk D, or a plurality of semiconductor lasers having different wavelengths may be mounted in different locations. May be. The liquid crystal lens element 64 may take the form of the first embodiment or the second embodiment described above. Therefore, description of the structure and manufacturing method of the liquid crystal lens element 64 and the operation principle will be omitted.
Furthermore, in the optical head device of the present invention, in addition to the optical components shown in FIG. 9, different optical components or mechanical components such as a diffraction grating, a hologram element, a polarization-dependent selection element, a wavelength-selective element, and a wavefront conversion unit are appropriately used. It can be applied in combination.

Next, the operation of the present invention will be described.
The linearly polarized light in the X direction emitted from the semiconductor laser 61 as the light source passes through the polarization beam splitter 92, then passes through the collimator lens 63, the liquid crystal lens element 64, and the quarter wavelength plate 65, and then converted into circularly polarized light. Then, the light is condensed on the first recording layer D1 or the second recording layer D2 provided in the optical disc D by the objective lens 66. After that, the light reflected from the optical disk D passes through the objective lens 66 and the quarter-wave plate 65 again, is converted into linearly polarized light in the Y direction, passes through the liquid crystal lens element 64 and the collimator lens 63, The light is reflected by the polarization beam splitter 62, given astigmatism by the cylindrical lens 67, and enters the photodetector 68.

Next, using the optical head device 60 equipped with the liquid crystal lens element 10 according to the first embodiment of the present invention as the liquid crystal lens element 64, information is recorded and / or reproduced on the recording layers D1 and D2 having different cover thicknesses. The operation to perform will be described below. However, in the following, it is assumed that the objective lens 66 is designed so that the aberration is minimized at an intermediate cover thickness between the first recording layer D1 and the second recording layer D2.
For example, when focusing on a recording layer with a cover thickness different from the design, spherical aberration proportional to the cover thickness difference obtained by subtracting the design thickness from the recording thickness of the cover thickness occurs, making it difficult to read and write information. This spherical aberration can be corrected by making light incident on the objective lens 66 into divergent light or convergent light obtained by adding a power component to a plane wave. In other words, the first recording layer D1 having a negative cover thickness difference generates a convergent light by adding a positive power, while the second recording layer D2 having a positive cover thickness difference adds a negative power. To convert to divergent light. Thereafter, if the light is condensed by the objective lens 95, the spherical aberration is corrected and information can be normally read and written.
(I) For recording and / or reproduction on the first recording layer D1 (cover thickness difference is negative):
In recording and / or reproduction on the first recording layer D1, as described above, the transparent wave 15, 16 and the transparent electrodes 17, 18 are arranged so that the transmitted wave front of the liquid crystal lens element 10 is a slightly condensed spherical wave. An AC voltage V _{+ 1} is applied between them. Then, the alignment directions of the liquid crystal layers 24 and 25 change, and as shown in FIG. 6A, positive power, that is, a transmission wavefront corresponding to a convex lens is obtained. Therefore, it is possible to correct the spherical aberration of the light condensed on the first recording layer D1.
(Ii) In the case of recording and / or reproduction on the second recording layer D2 (cover thickness difference is positive):
In recording and / or reproduction on the second recording layer D2, the AC voltage V _{− is} applied between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18 so that the transmitted wavefront of the liquid crystal lens element 10 is a slightly diverging spherical wave. Apply ₁ Then, the orientation directions of the liquid crystal layers 24 and 25 change, and as shown in FIG. 6C, a negative wave, that is, a transmitted wavefront corresponding to a concave lens is obtained. Accordingly, it is possible to correct the spherical aberration of the light condensed on the second recording layer D2.
As described above, the spherical aberration of the two recording layers having different cover thicknesses can be corrected by changing the voltage applied to the liquid crystal layer.

The liquid crystal lens element of the present invention has a spherical aberration regardless of the polarization incident on the liquid crystal lens element as long as the first and second Fresnel lens portions perform the same operation with respect to two linearly polarized lights orthogonal to each other. It can be corrected. However, in the situation where the manufacturing error of the liquid crystal lens element, for example, the thicknesses of the liquid crystal layers 24 and 25 are different, birefringence occurs, so that appropriate power cannot be obtained depending on the incident polarized light. Therefore, it is desirable that the polarization of the light incident on the liquid crystal lens element is linearly polarized light that matches the alignment direction of any liquid crystal layer provided in the Fresnel lens portion.
As described above, in the optical head device 60 illustrated in FIG. 9, the forward light that is the light incident on the optical disc D and the polarization of the return light that is the light reflected from the optical disc D are orthogonal to each other. Therefore, if the polarization of the forward light coincides with the liquid crystal alignment direction of the second Fresnel lens part, and if the polarization of the return light coincides with the liquid crystal alignment direction of the first Fresnel lens part, the spherical aberration is caused by the round-trip light. It can be corrected. In addition, it is preferable that the refractive indexes of the concave and convex portions 27 and 28 coincide with the ordinary light refractive index of the liquid crystal because the wavefront does not change with respect to the polarization component in the ordinary light refractive index direction.

Next, a case where variations occur in the cover thicknesses of the first and second recording layers D1 and D2 due to manufacturing variations of the optical disc D will be described.
By using the Fresnel lens portions 10A and 10B, the reference cover thickness difference between the first and second recording layers D1 and D2 can be accurately corrected. However, since the Fresnel lens portions 10A and 10B can only generate certain predetermined discrete power components, it is difficult to cope with individual cover thickness variations in which the cover thickness of the recording layer is different from the reference value. . On the other hand, it is difficult for the electrode lens part to obtain the same or larger power than the Fresnel lens part.
Therefore, in the liquid crystal lens element 10 of the present invention, spherical aberration caused by switching between the first recording layer and the second recording layer is corrected mainly by the discrete power generated by the Fresnel lens portion, It is convenient if the cover thickness variation is corrected so as to be corrected mainly by the continuous power generated by the electrode lens portion.

[Fourth Embodiment]
A configuration example of the liquid crystal lens element 70 according to the fourth embodiment of the present invention will be described below. FIG. 10 is a sectional view showing a fourth embodiment of the liquid crystal lens element of the present invention. In FIG. 10, the same components as those in FIG.
Compared with the liquid crystal lens element 10 according to the first embodiment of the present invention shown in FIG. 1, the liquid crystal lens element 70 according to the fourth embodiment serves as an electrode lens part as the electrode lens part of the first embodiment. The difference is that the electrode lens unit 10D (second electrode lens unit) is further added to 10C (first electrode lens unit).

In addition, the liquid crystal lens element 70 according to the present embodiment includes a Fresnel lens unit 70A in which the first Fresnel lens unit 10A and the second Fresnel lens unit 10B are integrated, a first electrode lens unit 10C, and a second electrode lens unit. The electrode lens unit 70B is integrated with 10D. The Fresnel lens unit 70A and the electrode lens unit 70B are separated from each other and are independently connected to the external signal power sources 30A and 30B, which is different from the liquid crystal lens element 10 according to the first embodiment. .
Other general configurations are the same as those of the liquid crystal lens element 10 according to the first embodiment, and the transparent substrates 13A, 13B, and 13C are the same as the transparent substrate 13.

  Here, similarly to the first electrode lens unit 10C, the second electrode lens unit 10D includes the fourth liquid crystal layer 26B sandwiched between the transparent substrates 13C and 14 and the seal 23B, and the fourth liquid crystal layer 26B. A transparent electrode 19B for applying a voltage to the composite electrode 20B and a composite electrode 20B are provided. The electrode lens unit 10D is continuously variable in focus by changing the substantial refractive index distribution of the liquid crystal layer 26B with respect to linearly polarized incident light in the Y direction according to the voltage distribution generated in the composite electrode 20B. Functions as a lens.

  The transparent electrodes 15, 17 and 16, 18 are connected to the external signal source 30A via electrode extraction portions 15A, 17A and 16A, 18A. On the other hand, the transparent electrodes 19 and 19B are connected to the external signal source 30B via the electrode extraction portion 19A. The composite electrode 20B has the same structure as the composite electrode 20, and, as shown in FIG. 3, low resistance electrodes 31 to 34 arranged concentrically around the optical axis, and uniform high resistance. A planar electrode 35 is provided. Among these, the low resistance electrodes 31 to 34 are connected to the external signal source 30B via the electrode extraction portions 31A to 34A.

  The fourth liquid crystal layer 26B of the second electrode lens unit 10D and the third liquid crystal layer 26 of the first electrode lens unit 10C have different liquid crystal alignment directions. That is, in the fourth liquid crystal layer 26B, the surface of the transparent electrode 19B and the composite electrode 20B is subjected to alignment treatment so that the extraordinary light refractive index is directed in the Y direction.

Therefore, the electrode lens unit 70B is a lens that is continuously variable in focus according to the voltage applied by the external signal source 30B regardless of the polarization state of the incident light. As a result, with the configuration of the liquid crystal lens element 70 according to the present embodiment, a lens function that is discretely variable in focus and a lens function that is continuously variable in focus are obtained regardless of the polarization state of incident light.
In the optical head device 60 shown in FIG. 9, when the liquid crystal lens element 70 is used in place of the liquid crystal lens element 64, the effects described in the third embodiment can be obtained. In particular, since the electrode lens unit 70B functions as a lens that can continuously focus on not only the incoming polarized light in the X direction but also the incoming polarized light in the Y direction on the backward path, the spherical aberration on the backward path is also effectively corrected. be able to. As a result, the accuracy of the focus servo is improved, and more stable recording / reproduction of a two-layer optical disc can be performed.

[Fifth Embodiment]
A configuration example of the liquid crystal lens element 80 according to the fifth embodiment of the present invention will be described below. FIG. 11 is a cross-sectional view showing a fifth embodiment of the liquid crystal lens element of the present invention. In FIG. 11, the same components as those in FIG.
The liquid crystal lens element 80 according to the present embodiment is different from the liquid crystal lens element 70 according to the fourth embodiment of the present invention shown in FIG. 10 in that the first Fresnel lens portion 10A and the second electrode lens portion 10D are integrated. A lens for the incident polarization in the X direction in which the liquid crystal lens unit 80A for return path that exhibits a lens function with respect to the incident polarization in the Y direction, the second Fresnel lens unit 10B, and the first electrode lens unit 10C are integrated. It consists of a forward liquid crystal lens portion 80B that exhibits its function. The backward liquid crystal lens unit 80A and the forward liquid crystal lens unit 80B are separated from each other and are independently connected to the external signal power supplies 30C and 30D. Other general configurations are the same as those of the liquid crystal lens element 70 according to the fourth embodiment.

  Therefore, the X-direction forward liquid crystal lens unit 80B functions with respect to incident polarized light in the X direction, and becomes a lens that is discretely and continuously variable in focus according to the voltage applied by the external signal source 30C. Further, the backward liquid crystal lens unit 80A functions for incident polarized light in the Y direction, and becomes a lens that is discretely and continuously variable in focus according to the voltage applied by the external signal source 30D.

Here, in the optical head device 60 shown in FIG. 9, when the liquid crystal lens element 80 is used instead of the liquid crystal lens element 64, the effects described in the fourth embodiment can be obtained.
Further, FIG. 12 shows an optical head device 90 having a configuration in which the forward liquid crystal lens unit 80B is disposed in the optical path of the optical head device and the backward liquid crystal lens unit 80A is disposed in the optical path of the backward path. Show. In FIG. 12, the same components as those in FIG.
In the optical head device 90 of FIG. 12, collimator lenses 63A and 63B are used for the forward path and the backward path, respectively, and a polarization beam splitter 62 is provided in the optical path between the collimator lenses 63A and 63B and the quarter-wave plate 65. The arrangement is different from the optical head device 60. Further, in the optical head device 90, a liquid crystal lens unit 80B having a lens action with respect to the forward linearly polarized light (polarized light in the paper) and a liquid crystal having a lens action with respect to the linearly polarized light in the backward path (polarized light perpendicular to the paper). The lens portion 80A is disposed between the collimator lenses 63B and 63A and the polarization beam splitter 62, respectively. As a result, compared to the case of the optical head device 60, there is a feature that high transmittance can be easily obtained because the liquid crystal layer having no lens action is not passed on the forward path and the return path.

[Sixth Embodiment]
Next, a configuration example of the liquid crystal lens element according to the sixth embodiment of the present invention will be described below.
In the liquid crystal lens element of this embodiment, the configuration of the Fresnel lens portion is different from those of the other embodiments.
That is, in the first Fresnel lens portion 10A and the second Fresnel lens portion 10B of the liquid crystal lens element shown in FIGS. 1, 10, and 11, the first liquid crystal layer 24 and the second liquid crystal layer 25 are negatively charged. A nematic liquid crystal having dielectric anisotropy is used. When the voltage is not applied to the liquid crystal layer, the orientation direction of the liquid crystal molecules is perpendicular to or substantially perpendicular to the substrate surface, and the refractive index n of the first uneven portion 27 and the second uneven portion 28 is. _F consists of uniform refractive index material of the same or close to the ordinary refractive index n _o of the liquid crystal layer. As described above, in order to make the alignment direction of the liquid crystal molecules be perpendicular to the substrate surface or an angle close to perpendicular to the substrate surface, a liquid crystal vertical alignment film may be formed on the substrate surface in contact with the liquid crystal layer.
On the other hand, in the ON state in which a voltage is applied to the liquid crystal layer, the alignment film surface is preferably subjected to an alignment treatment so that the alignment direction of the liquid crystal molecules is inclined in a specific direction. Specifically, the first liquid crystal layer 24 is aligned in the Y direction and the second liquid crystal layer 25 is aligned so that the liquid crystal molecules are inclined in the X direction.

With such a configuration, in the off state, the refractive index of the liquid crystal layer and the concavo-convex portion are almost the same regardless of the polarization state of the incident light, so that the transmitted wavefront does not change regardless of the shape of the concavo-convex portion. Further, since the difference in refractive index due to the difference in the wavelength dispersion of the refractive index between the liquid crystal layer and the uneven portion is slight, there is almost no change in the transmitted wavefront even if the wavelength of the incident light is changed. On the other hand, in the ON state, the alignment direction of the liquid crystal molecules changes according to the shape of the concavo-convex part and the applied voltage, and the liquid crystal layer substantially does not respond to incident light of linearly polarized light (that is, extraordinary light polarized light) in the aligned direction. The refractive index changes. As a result, the transmitted wavefront changes according to the applied voltage and the shape of the concavo-convex portion.
For example, when the center of the concavo-convex portion of the Fresnel lens is concave, in the off state, as shown in FIG. 6 (B), there is no change in the transmitted wavefront, and in the on state, it corresponds to a convex lens as shown in FIG. 6 (A). It can be a convergent transmitted wavefront. That is, by switching on / off of the applied voltage, a binary focus switching lens with no power and with power is obtained.
Since the first liquid crystal layer 24 and the second liquid crystal layer 25 are in the ON state and the XY in-plane projection components in the tilt direction of the liquid crystal molecules are orthogonal, the concave and convex portions 27 and 28 of the first and second Fresnel lens portions are If the shape and the layer thickness of the liquid crystal layers 24 and 25 are the same, and the same applied voltage is applied between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18 from an external signal source, the polarization state of incident light is affected. A single convergent wavefront. Note that the liquid crystal lens element having the Fresnel lens portion of this embodiment as a constituent element may have any configuration shown in FIG. 1, FIG. 10, or FIG. 11 in combination with the electrode lens portion.

Next, a single-layer high-density optical disc including the second recording layer D2 and the first recording layer D1 using an optical head device in which the liquid crystal lens element of this embodiment is mounted instead of the liquid crystal lens element 64 of FIG. The information recording / reproducing operation of the two-layer high-density optical disc including the second recording layer D2 will be described below. Here, the objective lens 66 is designed to minimize the aberration with respect to the second recording layer D2 having a cover thickness of 100 μm. The wavelength of the semiconductor laser 61 is in the 405 nm band. Other configurations are the same as those of the optical head device of the third embodiment.
During recording / reproduction of the second recording layer D2 having a cover thickness of 100 μm, the aberration performance of the objective lens 66 is maintained and the light collection performance is stable by applying no voltage to the Fresnel lens portion of the liquid crystal lens element and turning it off. Is realized. On the other hand, when recording / reproducing the first recording layer D1 having a cover thickness of 75 μm, a positive voltage is applied by applying a voltage between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18 of the Fresnel lens portion of the liquid crystal lens element. Convergent light with added power, spherical aberration is corrected, and as a result, stable light collection performance is realized.
In addition, spherical aberration that occurs due to variations in the cover thickness of the first recording layer D1 and the second recording layer D2 due to manufacturing variations of high-density optical discs is the same as in the third or fourth embodiment. Correction is performed by the continuous power generated according to the voltage applied to the unit.
The liquid crystal lens element of the present invention is arranged separately from the objective lens in order to correct the spherical aberration caused by the difference in the cover thickness by changing the focal length according to the applied voltage, Even when the objective lens is decentered from the liquid crystal lens element during tracking, there is an advantage that there is almost no deterioration of aberration. As a result, stable recording / reproduction of single-layer and double-layer high-density optical discs can be realized.
In the liquid crystal lens element of the third embodiment, the voltages applied between the transparent electrodes 15 and 16 and the transparent electrodes 17 and 18 of the Fresnel lens portion are switched to V ₊₁ , V ₊₀ , and V ₋₁ , respectively. The three-value focus switching lens function of “convex lens”, “no lens action”, and “concave lens” can be obtained. In the liquid crystal lens element of this embodiment, “no lens action”, “ A binary focus switching lens function of “convex lens” is obtained. Compared with the ternary focus switching of the third embodiment, the change in the optical path length between the electrodes, which is necessary for the binary focus switching of the present embodiment, is about half, so that the gap G between the transparent electrodes can be reduced. That is, since the layer thickness of the liquid crystal layer and the film thickness d of the concavo-convex portion can be reduced, the response speed at the time of switching the focus is increased. In addition, as the thickness d of the concavo-convex portion is thinner, the manufacturing process of the concavo-convex portion can be shortened and the alignment of the liquid crystal molecules on the surface of the concavo-convex portion is stabilized. There is an advantage that light is reduced and high efficiency is easily obtained.
In addition, in the arrangement of the optical head device that is incident on the liquid crystal lens element, in the case of the liquid crystal lens of the present embodiment, light of a wavelength of DVD or CD different from the wavelength used in the high-density optical disc is also in an off state when no voltage is applied. Since there is no change in the transmitted wavefront regardless of the wavelength of the incident light, it is preferable that the performance of the optical head device of DVD or CD is not deteriorated.

"Example 1"
Next, specific examples of the liquid crystal lens element 10 of the present invention shown in the first embodiment will be described below with reference to FIG.
First, a method for manufacturing the liquid crystal lens element 10 will be described.
A transparent conductive film (ITO film) is formed on one side or both sides of the transparent substrates 11 to 13 made of glass, and patterning is performed to make the transparent electrodes 15 to 19. Further, a SiON film, which is a uniform refractive index material having a refractive index n _S (= 1.52) and a relative dielectric constant ε _S (= 4), is formed on the transparent electrodes 16 and 18 with a film thickness d (= 2.9 μmm). Vapor deposition is performed. Next, the SiON film is processed by a photolithography technique and an etching technique so as to correspond to the shape of the graph F1 in FIG. 4, the cross-sectional shape is sawtooth, and rotational symmetry with respect to the optical axis (Z axis) of incident light is obtained. The uneven portions 27 and 28 as shown in FIG. 1 are formed.
On the other hand, after forming an ITO film having a sheet resistance value of 40Ω / □ on the surface of the transparent substrate 14 made of glass, low resistance electrodes 31 to 34 are formed by patterning as shown in FIG. Furthermore, after forming a tin oxide film having a sheet resistance value of 10 ⁶ Ω / □, patterning is performed to form a high-resistance planar electrode 35, thereby forming the composite electrode 20. Then, after applying and baking the liquid crystal aligning film which consists of polyimide on the surface of all the transparent substrates in which the electrode was formed, the surface of the transparent electrodes 15 and 16 is set to the Y-axis direction, the transparent electrodes 17-19 and the composite electrode 20 The surface is rubbed in the X-axis direction. Further, adhesives mixed with a gap control material having a diameter of 15 μm are printed and patterned on the surfaces of the transparent substrates 11, 12, 13 on which the transparent electrodes 15, 17, 19 are formed. 11 to 14 are overlapped and pressure-bonded to produce an empty cell having a transparent electrode interval of 15 μm.
Thereafter, nematic liquid crystal having positive dielectric anisotropy of ordinary light refractive index n _o (= 1.52) and extraordinary light refractive index n _e (= 1.70) is injected from the inlet (not shown) of the empty cell. The liquid crystal layers 24, 25, and 26 are used. Thereafter, the inlet is sealed with an ultraviolet curable resin, and then the conduction connecting means 29 is connected to obtain the liquid crystal lens element 10 shown in FIG.

The liquid crystal lens element 10 thus obtained is electrically connected to the external signal source 30 so that a voltage can be applied to the liquid crystal layers 24, 25 and 26. When increasing the applied voltage from 0V, the substantial refractive index of the rubbing direction of the liquid crystal layer 24 to 26 changes from _n e (= 1.70) to _n o (= 1.52). However, the effective voltage V _LC applied to the liquid crystal varies according to the shape of the concavo-convex portions 27 and 28 according to the equation (4), that is, depending on the location, and the phase difference φ _d generated by the liquid crystal lens element 10 is: Depending on the film thickness d _F of the concavo-convex portions 27 and 28, it changes as shown in equation (5).

Next, FIG. 13 is an explanatory diagram showing the Fresnel lens efficiency of the liquid crystal lens element 10 in the first embodiment. The horizontal axis of FIG. 13 is the voltage applied between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18 using the external signal source 30, and here, the electrode lens portion 10C, that is, the transparent electrode 19, the composite electrode The voltage between 20 is set to 0V.
In FIG. 1, when linearly polarized light in the X direction is incident, since the liquid crystal layer 24 is aligned in the Y direction, there is no substantial difference in refractive index from the concavo-convex portion 27, so that light is transmitted regardless of the applied voltage. To do. On the other hand, the phase difference generated in the liquid crystal layer 25 and the concavo-convex portion 28 varies depending on the voltage according to the equation (5) according to the film thickness d _F of the concavo-convex portion 28.
At an applied voltage of 1.35 V, n (V _LC [d _F ])> n _S , and the phase difference between the thinnest part and the thickest part of the concavo-convex part 28 is λ, as shown in FIG. In addition, the incident plane wave is converted into a wavefront that is slightly condensed as a + 1st order Fresnel diffraction wave. The + 1st order Fresnel diffraction efficiency becomes maximum at an applied voltage of 1.35 V as shown in graph A of FIG.
Similarly, at an applied voltage of 2.85 V, n (V _LC [d _F ]) <n _S , and the −1st-order Fresnel diffraction efficiency is maximum at an applied voltage of 2.85 V as shown in graph C of FIG. Become.
On the other hand, when the applied voltage is 1.74V, n (V _LC [d _F ]) ≈n _S , and the wavefront hardly changes, and the applied voltage is 1.74V as 0th-order Fresnel diffraction as shown in the graph B of FIG. It becomes the maximum in.
As described above, when the applied voltage is changed to 1.35V, 1.74V, and 2.85V, the liquid crystal lens element of the present invention acts as a “convex lens”, “no lens action”, and “concave lens”.
Next, if linearly polarized light in the Y direction is incident, the substantial refractive index of the liquid crystal layer 25 becomes n _o = n _S , so there is no lens action. Since the above-described concavo-convex portions 27 and 28 are the same, and the liquid crystal layers 24 and 25 have the same liquid crystal material and the alignment directions are orthogonal to each other, they are changed to 1.35V, 1.74V, and 2.85V as described above. And “convex lens”, “no lens action”, and “concave lens”.

Therefore, when the liquid crystal lens element of the present invention is used, the lens action can be switched according to the magnitude of the applied voltage for linearly polarized light in the X direction and the Y direction.
Next, without applying a voltage between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18, a voltage is applied between the electrode lens portion 10 </ b> C, that is, between the transparent electrode 19 and the composite electrode 20.
For example, when linearly polarized light equal to the alignment direction X direction of the liquid crystal layer 26 is incident, voltages applied to the low resistance electrodes 31 to 34 are V _A , V _B , V _C , and V _D, and 0 (V) <V _A An appropriate voltage satisfying <V _B <V _C <V _D is applied. Then, a maximum phase difference of 1.5λ can be obtained as shown in the graph β of FIG. Conversely, if an appropriate voltage satisfying 0 (V) <V _D <V _C <V _B <V _A is applied, a phase difference with a negative sign in the graph β of FIG. 7 can be obtained. Therefore, the wavefront including the power having the maximum phase difference of ± 1.5λ can be continuously changed by voltage control of the low resistance electrodes 31 to 34.
As described above, by using the liquid crystal lens element of the present invention, it is possible to obtain a focus switching Fresnel lens that works equally with orthogonal linearly polarized light. Further, by controlling the voltage applied to the composite electrode, the wavefront of linearly polarized light that matches the liquid crystal layer 26 can be continuously changed.

"Example 2"
Next, the liquid crystal lens element 10 shown in Example 1 is incorporated as the liquid crystal lens element 64 in the optical head device 60 shown in FIG. The cover thickness of the first recording layer D1 provided in the two-layer optical disc D is 75 μm, and the cover thickness of the second recording layer D2 is 100 μm.
In the optical head device 60, the light source 61 is a semiconductor laser having a wavelength of 405 nm, is collimated by the collimator lens 63, and enters the liquid crystal lens element 10. The objective lens 66 has an NA of 0.85, a pupil diameter of 3 mm, and is designed to minimize wavefront aberration with a cover thickness of 87.5 μm.

Here, when V ₀ = 1.74 V is applied, which is a case where the liquid crystal lens element does not exhibit a lens action, the wavefront aberration of the light condensed on each recording layer is affected by the spherical aberration proportional to the difference in cover thickness. Therefore, the light condensing performance is significantly deteriorated.
Next, when the voltage V ₊₁ = 1.35 V is applied between the transparent electrodes 15 and 16 and between the transparent electrodes 17 and 18 to collect light on the first recording layer D1, and between the transparent electrodes, V ₋₁ = 2. In the case of condensing on the second recording layer D2 by applying 85V, the spherical aberration is corrected to 0.01λrms or less, and the condensing performance is improved.

Next, in order to verify the correction performance of the spherical aberration due to the manufacturing error of the cover thickness of the double-layer optical disc D, the cover thickness of the double-layer optical disc D is formed to be 70 μm to 80 μm and 95 μm to 105 μm. The wavefront aberration is corrected by optimizing the voltage applied to the liquid crystal lens element 10 as appropriate. FIG. 14 is a diagram showing the dependence of the wavefront aberration on the cover thickness in the optical head device 60 at that time.
As shown in FIG. 14, in the liquid crystal lens element 10, the aberration is corrected by the Fresnel lens portions 10A and 10B designed to minimize the wavefront aberration when the cover thickness is 75 μm and 100 μm, respectively. A method that does not use the lens action of the lens unit 10C, graph B, is a case where the power generated by the electrode lens 10C is optimally adjusted.
As shown in FIG. 14, when the liquid crystal lens element 10 of the present invention is used, the magnitude of the wavefront aberration remaining in the cover thickness range of 70 to 80 μm or 95 to 105 μm can be reduced to λ / 30 rms or less. Even when manufacturing variations occur in the cover thickness, the wavefront aberration can be corrected and the light condensing performance can be maintained.

"Example 3"
Next, specific examples of the liquid crystal lens element 70 of the present invention shown in the sixth embodiment will be described below with reference to FIG. In addition, since the content of the component same as the liquid crystal lens element 10 is the same, description is abbreviate | omitted.
The main differences between the “example 3” and the liquid crystal lens element 10 (see FIG. 1) of “example 1” are the configurations of the first Fresnel lens portion 10A and the second Fresnel lens portion 10B, and the electrode lens portion. Is a point composed of the first electrode lens portion 10C and the second electrode lens portion 10D.

First, a method for manufacturing the first Fresnel lens unit 10A and the second Fresnel lens unit 10B will be described below.
For the first Fresnel lens portion 10A, transparent substrates 11 and 13A having transparent electrodes 15 and 18 made of ITO film on one side and transparent substrates 12 and 17 having transparent electrode films 16 and 17 made of ITO on both sides. The SiON film, which is a uniform refractive index material having a refractive index n _S (= 1.52) and a relative dielectric constant ε _S (= 4), is formed on the transparent electrodes 16 and 18 with a film thickness d (= 1.5 μmm). ) To form a film. Next, this SiON film has a sawtooth cross-sectional shape and rotational symmetry with respect to the optical axis (Z-axis) of incident light by a photolithography technique and an etching technique so as to correspond to the shape of the graph F1 in FIG. Concave and convex portions 27 and 28 as shown in FIG. 10 are formed. Further, after applying and baking a liquid crystal vertical alignment film (not shown) made of polyimide on the surfaces of the transparent electrodes 15 and 17 and the surfaces of the uneven portions 27 and 28, the surfaces of the transparent electrode 15 and the uneven portions 27 are set to Y. In the axial direction, the surface of the transparent electrode 17 and the concavo-convex portion 28 is rubbed in the X-axis direction. Further, an adhesive mixed with a 7 μm diameter gap control material is printed and patterned on the surface of the transparent substrate on which the transparent electrodes 16 and 18 are formed to form seals 21 and 22, and the transparent substrates 11, 12, and 13 A are overlaid. Together, pressure bonding is performed to produce an empty cell with a transparent electrode interval of 7 μm.
After that, nematic liquid crystal having negative dielectric anisotropy of ordinary light refractive index n _o (= 1.52) and extraordinary light refractive index n _e (= 1.70) is injected from the inlet (not shown) of the empty cell. The liquid crystal layers 24 and 25 are used. Thereafter, the injection port is sealed with an ultraviolet curable resin to form the Fresnel lens portion 70A shown in FIG. 10, and the external signal source 30A and the transparent electrode are electrically connected so that a voltage can be applied to the liquid crystal layers 24 and 25. To do.

Next, the first electrode lens unit 10C and the second electrode lens unit 10D will be described below with reference to FIGS.
The first electrode lens portion 10C and the second electrode lens portion 10D are mainly composed of a transparent substrate 13B having a transparent electrode 19 made of an ITO film on one side and a transparent electrode 19B made of an ITO film on one surface. A transparent substrate 14 formed with a composite electrode 20 formed on the other surface and a transparent substrate 13C formed with a composite electrode 20B on one surface is provided.
Regarding the production of the first electrode lens portion 10C and the second electrode lens portion 10D, in particular, the composite electrodes 20 and 20B are formed with ITO films having a sheet resistance value of 40Ω / □ on the transparent substrates 14 and 13C. Then, patterning is performed to form low resistance electrodes 31 to 35 (see FIG. 3), and a tin oxide film having a sheet resistance value of 10 ⁶ Ω / □ is formed, followed by patterning to form a high resistance flat electrode 35. (See FIG. 3). Further, after applying and baking a liquid crystal horizontal alignment film made of polyimide on the surfaces of the transparent electrodes 19 and 19B and the above-described composite electrodes 20 and 20B, the surfaces of the transparent electrode 19 and the composite electrode portion 20 are arranged in the X-axis direction. The surfaces of the transparent electrode 19B and the composite electrode portion 20B are rubbed and aligned in the Y-axis direction. Thereafter, nematic liquid crystal having positive dielectric anisotropy of ordinary refractive index n _o (= 1.52) and extraordinary refractive index n _e (= 1.70) is injected from the inlet (not shown) of the empty cell. The liquid crystal layers 26 and 27 are used. Thereafter, the injection port is sealed with an ultraviolet curable resin to form the electrode lens portion 70B shown in FIG. 10, the external signal source 30B and each electrode are electrically connected, and the same voltage can be applied to the liquid crystal layers 26 and 27. Like that.
In the liquid crystal lens element 70 manufactured as described above, when the AC applied voltage generated from the external signal sources 30A and 30B is increased from 0V, the liquid crystal layers 24 and 25 are vertically aligned liquid crystals having negative dielectric anisotropy. in, since the liquid crystal layer 26 and 27 are horizontally aligned liquid crystal having a positive dielectric anisotropy, the substantial refractive index of the rubbing direction of the liquid crystal layer 24 and 25 _n o (= 1.52) from _{n e} (= 1.70) to change substantial refractive index of the rubbing direction of the liquid crystal layer 27 changes from _n e (= 1.70) to _n o (= 1.52).

FIG. 15 is an explanatory diagram showing the Fresnel lens efficiency of the liquid crystal lens element 70 in “Example 3”. The horizontal axis in FIG. 15 is the voltage applied between the transparent electrodes 15 and 16 and the transparent electrodes 17 and 18 of the Fresnel lens unit 70A using the external signal source 30A. Here, the electrode lens unit 70B includes Suppose no voltage is applied.
(I) In the off state when no voltage is applied, the refractive index of the liquid crystal layer and the concavo-convex portion are the same regardless of the polarization state of the incident light, so the transmitted wavefront is unchanged. That is, “no lens action”.
(II) On the other hand, in the on state at the time of voltage application, when linearly polarized light in the X direction is incident, the liquid crystal layer 24 is aligned in the Y direction, so that a substantial difference in refractive index from the uneven portion 27 does not occur. The light is transmitted regardless of the applied voltage. On the other hand, since the liquid crystal layer 25 is oriented in the X direction, the phase difference generated in the liquid crystal layer 25 and the concavo-convex portion 28 varies depending on the applied voltage according to the film thickness d _F of the concavo-convex portion 28. When the applied voltage is 3.5 V, the phase difference between the thinnest part and the thickest part of the concavo-convex part is λ, and the incident plane wave has the highest efficiency of the + 1st order Fresnel diffraction wave as shown in FIG. And converted into a convergent wavefront equivalent to a convex lens.
Further, when linearly polarized light in the Y direction is incident, the liquid crystal layer 25 is aligned in the X direction, so that no substantial difference in refractive index from the concavo-convex portion 28 occurs, so that light is transmitted regardless of the applied voltage. On the other hand, since the liquid crystal layer 24 is aligned in the Y direction, the phase difference generated in the liquid crystal layer 24 and the concavo-convex portion 27 varies depending on the applied voltage according to the film thickness d _F of the concavo-convex portion 27. When the applied voltage is 3.5 V, the phase difference between the thinnest part and the thickest part of the concavo-convex part is λ, and the incident plane wave has the highest efficiency of the + 1st order Fresnel diffraction wave as shown in FIG. And converted into a convergent wavefront equivalent to a convex lens. Note that rms of the applied voltage unit Vrms in FIGS. 13 and 15 means an AC effective voltage.

As described above, when the applied voltage is switched between 0 V (off state) and 3.5 V (on state), the liquid crystal lens element of the present invention acts as “no lens action” and “convex lens”. Therefore, when the liquid crystal lens element of the present invention is used, the lens action can be switched according to on / off of the applied voltage with respect to linearly polarized light in the X direction and Y direction, that is, regardless of the polarization state of incident light. .
Next, without applying a voltage between the transparent electrodes 15 and 16 of the Fresnel lens unit 70A and between the transparent electrodes 17 and 18, between the transparent electrode 19 and the composite electrode 20 of the electrode lens unit 70B and the transparent electrode 19B. A voltage is applied between the composite electrodes 20B. Since the orientation directions of the liquid crystal layer 26 and the liquid crystal layer 26B are orthogonal to each other, the wavefront including power is continuously changed by voltage control of the low resistance electrodes 31 to 34 with respect to the linearly polarized light in the X direction and the Y direction. can do.
As described above, by using the liquid crystal lens element 70 of the present invention, it is possible to obtain a binary focus switching lens that operates regardless of the polarization state of incident light. In addition, by controlling the voltage applied to the electrode lens unit, the wavefront including the power that acts regardless of the polarization state of the incident light can be continuously changed.

Next, the liquid crystal lens element 70 of the present invention was placed in place of the liquid crystal lens element 64 of the optical head device 90 of FIG. 9 and used for recording / reproduction of a single-layer high-density optical disk and a double-layer high-density optical disk.
(I) First, when recording / reproducing the second recording layer D2 having a cover thickness of 100 μm, the Fresnel lens portion 70A of the liquid crystal lens element 70 is turned off, and when recording / reproducing the first recording layer D1 having a cover thickness of 75 μm. Applies a voltage of 3.5 V to the Fresnel lens portion 70A of the liquid crystal lens element 70 from the external signal source 30A to switch it to the on state.
Here, FIG. 14A shows the calculation result of the remaining RMS wavefront aberration when the cover thicknesses of the first recording layer D1 and the second recording layer D2 vary by ± 5 μm. When the cover thickness is 100 μm and 75 μm, the RMS wavefront aberration is 0.01 λrms or less, and when the cover thickness varies ± 5 μm, an RMS wavefront aberration of about 0.05 λrms occurs.
(II) Further, according to the variation of the cover thickness of the first recording layer D1 and the second recording layer D2, a voltage is applied from the external signal source 30B to the electrode lens portion 70B of the liquid crystal lens element 70 to correct the aberration. FIG. 14B shows the calculation result of the remaining RMS wavefront aberration in this case. Even when the cover thickness fluctuates by ± 5 μm, it can be reduced to the occurrence of RMS wavefront aberration of 0.03 λrms or less.
(III) Further, even when the objective lens 66 is decentered about ± 0.3 mm from the liquid crystal lens element 70 during tracking, the RMS wavefront aberration shown in FIG. 14 is hardly deteriorated. Therefore, by using the optical head device 90 on which the liquid crystal lens element 70 of the present invention is mounted, stable recording / reproduction of single-layer and double-layer high-density optical discs is realized.

  The present invention is not limited to the embodiment described above, and can be implemented in various forms without departing from the gist of the present invention.

  The liquid crystal lens element of the present invention can be used as a focal length switching lens in which the focal length is switched discretely by switching the applied voltage, and the focal length can be changed continuously according to the magnitude of the applied voltage. Can be used with a lens. In particular, in recording and / or reproduction of an optical disc having two information recording layers having different cover thicknesses, the liquid crystal lens element and the objective lens can be used as a liquid crystal lens element that corrects spherical aberration including a generated power component. Since no aberration occurs when the lens is decentered, the liquid crystal lens element can be arranged away from the objective lens. Furthermore, it can be used for an optical head device or the like as a small unit integrated with a light source, a photodetector, a beam splitter, or the like.

1 is a cross-sectional view showing a liquid crystal lens element according to a first embodiment of the present invention. 1 is a top view of a Fresnel lens portion of a liquid crystal lens element according to a first embodiment of the present invention. 1 is a top view of an electrode lens portion of a liquid crystal lens element according to a first embodiment of the present invention. 2 is a graph showing a phase difference of a transmitted wavefront generated by the liquid crystal lens element of the present invention, wherein P1 and P2 are graphs in which the horizontal axis is a radius r and the phase difference is expressed in units of wavelength λ, and F1 and F2 are P1 and P2. A graph showing a phase difference of zero or more and λ or less by adding or subtracting an integral multiple of wavelength λ. The enlarged view of the Fresnel lens part in sectional drawing of the liquid-crystal lens element of this invention. It is sectional drawing which shows an effect | action when the applied voltage to the Fresnel lens part of the liquid crystal lens element of this invention is switched, Comprising: (A) is a convergent transmission wave front at the time of applied voltage V _{+ 1} , (B) is the applied voltage V. _A transmitted wavefront having no wavefront change at ₀ and (C) shows a divergent transmitted wavefront at an applied voltage V- ₁ . FIG. 4 is a schematic diagram showing a phase difference distribution generated by the electrode lens portion of the liquid crystal lens element of the present invention, where α is a target phase difference and β is a phase difference generated by the electrode lens portion. (A), (B) is a top view of the composite electrode which comprises the electrode lens part of the liquid-crystal lens element based on the 2nd Embodiment of this invention, respectively. The block diagram which shows an example of the optical head apparatus of this invention. Sectional drawing which shows 4th Embodiment of the liquid-crystal lens element of this invention. Sectional drawing which shows 5th Embodiment of the liquid-crystal lens element of this invention. The schematic diagram which shows another example of the optical head apparatus of this invention. Explanatory drawing which shows the Fresnel lens efficiency of the liquid-crystal lens element in the optical head apparatus of this invention. Explanatory drawing which shows the cover thickness dependence of the wavefront aberration in the optical head apparatus of this invention. Explanatory drawing which shows the Fresnel diffraction efficiency of the liquid-crystal lens element in a 3rd Example which is 6th Embodiment of the liquid-crystal lens element of this invention. The block diagram which shows the conventional optical head apparatus by which the movable lens group was mounted as a spherical aberration correction element. Sectional drawing which shows the structural example of the conventional liquid crystal lens. Sectional drawing which shows the structural example of the conventional liquid crystal diffraction lens.

Explanation of symbols

10, 70, 80 Liquid crystal lens element 10A First Fresnel lens part 10B Second Fresnel lens part 10C (First) electrode lens part 10D (Second) electrode lens part 11, 12, 13, 13A, 13B, 13C, 14 Transparent substrate 15, 16, 17, 18, 19, 19B Transparent electrode 15A-18A Electrode extraction part 19A Electrode extraction part 20, 20B Composite electrode 21, 22, 23, 23B Seal 24, 25, 26, 26B Liquid crystal layer 27, 28 Uneven portion 29 Conducting connection means 30, 30A, 30B, 30C, 30D External signal source 31-34 Low resistance electrode 31A-34A Electrode extraction portion 35 High resistance planar electrode 40, 50 Composite electrode 45, 55 High resistance planar electrode 41-44, 51-54 Low resistance electrode 61 Semiconductor laser 62 Polarizing beam splitter 63 Collimator lens 4 Liquid Crystal Lens Element 65 Quarter Wave Plate 66 Objective Lens 67 Cylindrical Lens 68 Photodetector 70A Fresnel Lens 70B Electrode Lens 80A Return Liquid Crystal Lens 80B Forward Liquid Crystal Lens D Optical Disc D1 First Recording Layer D2 First 2 recording layers

Claims (15)

A liquid crystal lens element that changes a focal length of light transmitted through the liquid crystal according to a magnitude of a voltage applied to the liquid crystal,
A first Fresnel lens part, a second Fresnel lens part, and an electrode lens part;
The first Fresnel lens unit includes a first liquid crystal layer sandwiched between a pair of transparent substrates, opposing electrode pairs respectively disposed on the surface of the transparent substrate to apply a voltage to the first liquid crystal layer, A first concavo-convex portion having a sawtooth-like cross-sectional shape having rotational symmetry with respect to the optical axis of light or a cross-sectional shape approximating a sawtooth like a step shape, and formed of a transparent material on at least one upper surface of the opposed electrode pair; With
The second Fresnel lens unit includes a second liquid crystal layer sandwiched between a pair of transparent substrates, opposing electrode pairs respectively disposed on the surface of the transparent substrate to apply a voltage to the second liquid crystal layer, A sawtooth-like cross-sectional shape having rotational symmetry with respect to the optical axis of the light, or a second concavo-convex portion formed of a transparent material on the upper surface of at least one of the opposing electrode pairs; With
The electrode lens unit is disposed opposite to the surface of the transparent substrate in order to apply a voltage to the third liquid crystal layer sandwiched between a pair of transparent substrates, and at least one of them is a low resistance An electrode pair that is a composite electrode composed of an electrode and a high-resistance planar electrode;
The first, second, and third liquid crystal layers are nematic liquid crystals that are aligned in parallel when no voltage is applied or when a voltage is applied, and the normal refractive index direction of the first liquid crystal layer, the second liquid crystal layer, and the third liquid crystal layer A liquid crystal lens element characterized in that an extraordinary refractive index direction of a liquid crystal layer coincides. Either one of the opposing electrode pairs installed in the electrode lens unit is the composite electrode,
2. The liquid crystal lens element according to claim 1, wherein the composite electrode includes a high-resistance flat electrode and a plurality of concentric low-resistance electrodes centered on the optical axis of the light. Both of the opposing electrodes installed in the electrode lens unit are the composite electrodes,
The one composite electrode includes a high resistance planar electrode and a plurality of low resistance electrodes arranged in a stripe pattern, and the other composite electrode includes an arrangement direction of the high resistance planar electrode and the low resistance electrode. The liquid crystal lens element according to claim 1, comprising a plurality of low-resistance electrodes arranged in stripes in a direction perpendicular to each other.   4. The liquid crystal lens element according to claim 1, wherein a refractive index of the transparent material forming the first and second concavo-convex portions is equal to an ordinary refractive index of the first liquid crystal layer and the second liquid crystal layer. The first Fresnel lens part, the second Fresnel lens part and the electrode lens part are laminated and integrated,
5. The liquid crystal lens according to claim 1, wherein the first, second, and third liquid crystal layers are respectively installed in a gap between three substrates formed by four transparent substrates facing each other. element.   6. The liquid crystal lens element according to claim 5, wherein a phase plate whose phase difference with respect to the wavelength of light is an odd multiple of [pi] / 2 is integrated. The electrode lens portion of the liquid crystal lens element includes a first electrode lens portion and a second electrode lens portion,
The first electrode lens unit is disposed on the surface of the transparent substrate in order to apply a voltage to the third liquid crystal layer sandwiched between a pair of transparent substrates, and at least one of them. An opposing electrode pair that is a composite electrode composed of a low-resistance electrode and a high-resistance planar electrode;
The second electrode lens unit is disposed on the surface of the transparent substrate in order to apply a voltage to the fourth liquid crystal layer sandwiched between the pair of transparent substrates, and at least one of them is low An opposing electrode pair that is a composite electrode composed of a resistance electrode and a high-resistance planar electrode;
The first, second, third, and fourth liquid crystal layers are nematic liquid crystals that are aligned in parallel when no voltage is applied or when a voltage is applied, and the normal refractive index direction of the first liquid crystal layer and the second and third liquid crystals. 2. The liquid crystal lens element according to claim 1, wherein the extraordinary refractive index direction of the layer and the ordinary refractive index direction of the fourth liquid crystal layer coincide with each other. One of the opposing electrodes installed in the electrode lens unit is the composite electrode,
The liquid crystal lens element according to claim 7, wherein the composite electrode includes a high-resistance planar electrode and a plurality of concentric low-resistance electrodes centered on the optical axis of the light. The opposing electrodes installed in the electrode lens part are both composite electrodes,
One of the composite electrodes is composed of a high resistance flat electrode and a plurality of low resistance electrodes arranged in a stripe pattern,
8. The liquid crystal lens element according to claim 7, wherein the other composite electrode includes a plurality of low resistance electrodes arranged in a stripe shape in a direction orthogonal to an arrangement direction of the high resistance planar electrode and the low resistance electrode.   10. The liquid crystal lens according to claim 7, wherein a refractive index of the transparent material forming the first and second concavo-convex portions is equal to an ordinary refractive index of the first liquid crystal layer and the second liquid crystal layer. element.   The first Fresnel lens part and the second Fresnel lens part are laminated and integrated, and the first electrode lens part and the second electrode lens part are laminated and integrated. Item 11. A liquid crystal lens element according to Item 7. A light source;
An objective lens for condensing the light emitted from the light source on the optical recording medium;
A photodetector for detecting light that is collected and reflected on the optical recording medium;
7. An optical head device comprising: the liquid crystal lens element according to claim 1 disposed in an optical path between the light source and the objective lens.   On the optical path of light from the light source toward the optical recording medium, the polarization direction of the light from the light source incident on the liquid crystal lens element coincides with the extraordinary refractive index direction of the third liquid crystal layer of the liquid crystal lens element. The optical head device according to claim 12.   The liquid crystal lens according to any one of claims 7 to 11, wherein a liquid crystal lens is provided in a light source, an objective lens for condensing light emitted from the light source on an optical recording medium, and an optical path between the light source and the objective lens. An optical head device comprising an element. A light source, an objective lens for condensing the light emitted from the light source on the information recording layer of the optical recording medium, a photodetector for receiving the reflected light of the information recording layer, and a light beam traveling in the forward direction from the light source to the optical recording medium; In an optical head device including at least a beam splitter that separates a reflected light beam from the information recording layer and a light beam in a return path toward the photodetector,
A forward liquid crystal lens element in which the first Fresnel lens part and the second electrode lens part are laminated and integrated, and a backward liquid crystal in which the second Fresnel lens part and the first electrode lens part are laminated and integrated. 11. The liquid crystal lens element according to claim 7, wherein the forward liquid crystal lens element is disposed in an optical path between the light source and the beam splitter, and the backward liquid crystal lens element is disposed between the beam splitter and the photodetector. An optical head device arranged in an optical path.

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