Liquid crystal optical element and optical pickup apparatus

The present invention is directed to the provision of an optical pickup apparatus in which a liquid crystal optical element constructed by combining an aberration correcting liquid crystal panel and an nλ/4 liquid crystal panel in an integral fashion is mounted in a tilted position. More specifically, the invention provides a liquid crystal optical element includes a first liquid crystal layer having a first rubbing direction and for correcting aberration, a second liquid crystal layer having a second rubbing direction and combined with the first liquid crystal layer in an integral fashion, and a transparent electrode for generating a potential difference across the second liquid crystal layer in order to control an amount of phase difference for the light beam passing through the second liquid crystal layer, and wherein the angle (ψ) that the second rubbing direction makes with the first rubbing direction is determined in accordance with the tilt angle (α) so that the second liquid crystal layer functions as an nλ/4 plate. The invention also provides an optical pickup apparatus incorporating such a liquid crystal optical element.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal optical element and an optical pickup apparatus, and more specifically to a liquid crystal optical element having an aberration correcting function, as well as a function of an nλ/4 plate, and an optical pickup apparatus incorporating such a liquid crystal optical element.

BACKGROUND OF THE INVENTION

For a liquid crystal panel that functions as a quarter-wave plate, it is known to provide a configuration in which the liquid crystal panel is tilted with respect to the optical axis by an angle equal to the angle (pretilt angle) that the long axis direction of liquid crystal molecules makes with the glass substrate (refer, for example, to patent document 1). By tilting the liquid crystal panel in this manner, the long axis direction of the liquid crystal molecules can be oriented at right angles to the optical axis, and the liquid crystal panel can thus be made to function as a perfect quarter-wave plate.

It is also known to provide a liquid crystal panel in which two liquid crystal layers are formed using three glass substrates, one liquid crystal layer being formed as an aberration correcting layer and the other as a quarter-wave plate (refer, for example, to patent document 2). In the liquid crystal panel described in patent document 2, the rubbing direction of the liquid crystal layer formed as the quarter-wave plate is oriented at an angle of 45 degrees relative to the rubbing direction of the aberration correcting liquid crystal layer, and the liquid crystal panel thus fabricated is arranged perpendicularly relative to the optical axis.Patent document 1: Japanese Patent Publication No. 3142251 (page 3 and FIG. 2)Patent document 2: Japanese Unexamined Patent Publication No. 2001-34996 (page 29 and FIG. 21).

SUMMARY OF THE INVENTION

If the aberration correcting liquid crystal panel is arranged perpendicularly relative to the optical axis, a problem arises in which light emitted from a light source is reflected by the liquid crystal panel directly back to the light source, causing the light rays to interfere with each other and resulting in the generation of noise, and hence an inability to obtain light of stable intensity from the light source. Accordingly, the aberration correcting liquid crystal panel needs to be tilted at a certain angle with respect to the optical axis. Further, when tilting the aberration correcting liquid crystal panel, it must be tilted in a direction perpendicular to its rubbing direction (the long axis direction of the liquid crystal molecules). If it is not tilted in such a direction, the aberration correcting liquid crystal panel will cause the direction of polarization of light to rotate, and thus the aberration correcting liquid crystal panel will become unable to perform its intended function for the light contained in the effective light beam.

On the other hand, in the λ/4 liquid crystal panel, a pretilt angle is provided between the long axis direction of the liquid crystal molecules and the direction parallel to the transparent glass substrate. Accordingly, if the λ/4 liquid crystal panel is not used by tilting it so as to reduce the pretilt angle to zero, there arises the problem that, because of its incidence angle dependence, the liquid crystal panel does not function as an accurate λ/4 plate.

Consider a liquid crystal optical element constructed by combining in an integral fashion the aberration correcting liquid crystal panel and the λ/4 liquid crystal panel whose rubbing direction is oriented at 45 degrees relative to the rubbing direction of the aberration correcting liquid crystal panel; if this liquid crystal optical element is tilted, both the aberration correcting liquid crystal panel and the λ/4 liquid crystal panel will tilt in the same direction as a matter of course. However, the rubbing direction of the aberration correcting liquid crystal panel and the direction that reduces the pretilt angle of the λ/4 liquid crystal panel to zero cannot be the same. As a result, the liquid crystal optical element constructed by combining the aberration correcting liquid crystal panel and the λ/4 liquid crystal panel in an integral fashion cannot be used in a tilted position in an optical pickup apparatus. The reason that the rubbing direction of the λ/4 liquid crystal panel is oriented at 45 degrees relative to the rubbing direction of the aberration correcting liquid crystal panel is that by so orienting the rubbing direction, the prescribed linearly polarized light passed through the aberration correcting liquid crystal panel can be converted to nearly perfect circularly polarized light by the λ/4 liquid crystal panel.

Accordingly, it is an object of the present invention to provide a liquid crystal optical element and an optical pickup apparatus that can resolve the above problem.

It is also an object of the present invention to provide an optical pickup apparatus in which a liquid crystal optical element constructed by combining an aberration correcting liquid crystal panel and a λ/4 liquid crystal panel in an integral fashion is mounted in a tilted position.

An optical pickup apparatus according to the present invention includes a light source for emitting a light beam, a liquid crystal optical element constructed by combining in an integral fashion a first liquid crystal layer having a first rubbing direction and for correcting aberration and a second liquid crystal layer having a second rubbing direction, the liquid crystal optical element being mounted tilted at a prescribed angle with respect to an optical axis of the light beam, a transparent electrode for generating a potential difference across the second liquid crystal layer in order to control an amount of phase difference for the light beam passing through the second liquid crystal layer, and an objective lens for focusing the light beam passed through the liquid crystal optical element, and wherein the angle that the second rubbing direction makes with the first rubbing direction is determined in accordance with the tilt angle so that the second liquid crystal layer functions as an nλ/4 plate.

Preferably, in the optical pickup apparatus according to the present invention, the second rubbing direction of the second liquid crystal layer is determined in accordance with the tilt angle of the liquid crystal optical element and the potential difference so that the second liquid crystal layer functions as an nλ/4 plate.

Further preferably, in the optical pickup apparatus according to the present invention, the second rubbing direction of the second liquid crystal layer is determined in accordance with the tilt angle of the liquid crystal optical element, the potential difference, and an operating temperature range so that the second liquid crystal layer functions as an nλ/4 plate.

A liquid crystal optical element according to the present invention includes a first liquid crystal layer having a first rubbing direction and for correcting aberration, a second liquid crystal layer having a second rubbing direction and combined with the first liquid crystal layer in an integral fashion, and a transparent electrode for generating a potential difference across the second liquid crystal layer in order to control an amount of phase difference for the light beam passing through the second liquid crystal layer, and wherein the angle that the second rubbing direction makes with the first rubbing direction is determined in accordance with the tilt angle so that the second liquid crystal layer functions as an nλ/4 plate.

Preferably, in the liquid crystal optical element according to the present invention, the angle that the second rubbing direction makes with the first rubbing direction is determined in accordance with the tilt angle and the potential difference so that the second liquid crystal layer functions as an nλ/4 plate.

Preferably, in the optical pickup apparatus and the liquid crystal optical element according to the present invention, the first liquid crystal layer corrects coma, spherical aberration, or astigmatism.

Preferably, in the optical pickup apparatus and the liquid crystal optical element according to the present invention, the first liquid crystal layer and the second liquid crystal layer are provided alternately between three transparent substrates.

According to the present invention, as the liquid crystal optical element is constructed by combining the aberration correcting liquid crystal panel and the nλ/4 liquid crystal panel in an integral fashion, the aberration correcting liquid crystal panel and the nλ/4 liquid crystal panel can be arranged in a single setup procedure.

Further, according to the present invention, using the liquid crystal optical element constructed by combining the aberration correcting liquid crystal panel and the nλ/4 liquid crystal panel in an integral fashion, not only can reflections be prevented from occurring at the aberration correcting liquid crystal panel, but at the same time, the nλ/4 liquid crystal panel can be made to function as an accurate nλ/4 plate.

Furthermore, according to the present invention, the angle Ψ that the rubbing direction of the second liquid crystal layer makes with the rubbing direction of the first liquid crystal layer for optimum operation of the nλ/4 liquid crystal panel can be determined based on the tilt angle α, etc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An optical pickup apparatus and a liquid crystal optical element according to the present invention will be described below with reference to the drawings. It should, however, be noted that the present invention is not limited by the description given herein, but embraces the inventions described in the appended claims and their equivalents.

FIG. 1is a schematic diagram showing the configuration of an optical apparatus1which includes a liquid crystal optical element and an optical pickup apparatus according to the present invention.

The optical apparatus1, which comprises the optical pickup apparatus10, a connector20, and a driving unit30, is an apparatus for writing and reading data on a recording medium4such as a DVD or CD.

In the optical pickup apparatus10, a light beam (650 nm) emitted from a light source11constructed from a semiconductor laser or the like is converted by a collimator lens12into a substantially parallel beam of light which passes through a polarization beam splitter13and enters the liquid crystal optical element100. The light passed through the liquid crystal optical element100is focused by an objective lens14(numerical aperture NA=0.65) onto the track surface of the recording medium4. As shown inFIG. 1, the light beam has an effective diameter2and an optical axis3. The tracking motion of the objective lens14is controlled by a servo driving mechanism15so that the spot focused by the objective lens14is accurately positioned on the track.

The light beam reflected from the track of the recording medium4again passes through the objective lens14and the liquid crystal optical element100, and enters the polarization beam splitter13where the direction of the light beam is changed and the light beam is focused by a converging lens16onto a light detector17. The light beam, when reflected at the recording medium4, is amplitude-modulated by the information (pit) recorded on the track surface of the recording medium4.

The driving unit30includes a light source driving circuit32for driving the light source11, a signal generating circuit33which outputs a light intensity signal (RF) based on a light detection signal supplied from the light detector17, a liquid crystal optical element driving circuit34for driving the liquid crystal optical element100, a servo driving circuit35for driving the servo driving mechanism15, a disk motor36for rotating the recording medium4, a motor driving circuit37for driving the disk motor36, and a controller31, containing a RAM, ROM, CPU, etc., for controlling the entire operation.

The liquid crystal optical element100, as will be described later, is constructed by combining, in an integral fashion, a first liquid crystal layer110that functions as an aberration correcting plate and a second liquid crystal layer120that functions as an nλ/4 plate, and is driven by a supply voltage supplied from the liquid crystal optical element driving circuit34. Here, the liquid crystal optical element driving circuit34may be incorporated in the liquid crystal optical element100to be described later.

Based on the light intensity signal generated by the signal generating circuit33, the controller31reads information recorded on the recording medium4by rotating the recording medium4by means of the disk motor36while controlling the light source driving circuit32and the servo driving circuit35.

Further, the controller31modulates the intensity of the light beam emitted from the light source11in accordance with a data signal to be written, and illuminates the recording medium4with the modulated light beam. On the track surface of the recording medium4to which the data is to be written, the refractive index or color of a thin film contained in the disk changes or a pit is formed in accordance with the intensity of the light beam, thus accomplishing the data write. The intensity modulation of the light beam is performed, for example, by the light source driving circuit32modulating the current to be supplied to the semiconductor laser device used as the light source11.

The optical pickup apparatus10is connected to the driving unit30via the connector20.

FIG. 2is a diagram schematically showing the structure of the liquid crystal optical element100.

FIG. 2(a) shows a cross-sectional view of the liquid crystal optical element100, andFIG. 2(b) shows a plan view of the liquid crystal optical element100as viewed from the light exit side.

As shown inFIG. 2(a), the liquid crystal optical element100is constructed by stacking a first transparent substrate101, a second transparent substrate102, and a third transparent substrate103one on top of another. The liquid crystal optical element100further includes the first liquid crystal layer110as an aberration correcting plate, which is sandwiched between the first and second transparent substrates101and102, and the second liquid crystal layer120as an nλ/4 plate, which is sandwiched between the second and third transparent substrates102and103.

A flexible board140is connected to the first and second liquid crystal layers110and120so that a prescribed voltage is applied to each individual liquid crystal layer from the liquid crystal optical element driving circuit34.

The first liquid crystal layer110is sealed between the first and second transparent substrates101and102by means of a sealing member130and a stopping member132. Likewise, the second liquid crystal layer120is sealed between the second and third transparent substrates102and103by means of a sealing member131and a stopping member133.

As shown inFIG. 2(b), in the liquid crystal optical element100, the rubbing direction111of the first liquid crystal layer110and the rubbing direction121of the second liquid crystal layer120are oriented at an angle ψ relative to each other. The method of determining the angle ψ will be described later.

For explanatory purposes, X-axis, Y-axis, and Z-axis are set relative to the liquid crystal optical element100, as shown inFIG. 2. The Z-axis indicates the direction perpendicular to the upper surface of the first transparent substrate101. In the optical pickup apparatus of the present invention, the liquid crystal optical element100is tilted at an angle α with respect to the optical axis3(which coincides with the direction of the incident light) in order to prevent the light beam emitted from the light source11from being reflected by the liquid crystal optical element100back toward the light source11. It is assumed here that the angle α at which the liquid crystal optical element100is mounted in the optical pickup apparatus10is predetermined.

When tilting the first liquid crystal layer110that functions as the aberration correcting plate, if the liquid crystal layer is not tilted in a direction perpendicular to its rubbing direction111, the linearly polarized light entering the aberration correcting liquid crystal panel will emerge from it as elliptically polarized light. Accordingly, in the present embodiment, when the rubbing direction111of the first liquid crystal layer110is parallel to the X-axis, the liquid crystal optical element100is tilted by the angle α in such a manner as to turn the X-axis about the Y-axis toward the right in the figure. Preferably, the angle α is approximately equal to the pretilt angle of the first liquid crystal layer110, for example, about 3 degrees, and its direction is the direction that cancels the pretilt angle of the first liquid crystal layer110. That is, the angle that the Z-axis makes with the optical axis3extending parallel to the incident light is the angle α. In the case ofFIG. 2(a), the liquid crystal optical element100is tilted to the right in the figure, but alternatively, it may be tilted to the left in the figure. For convenience, the angle by which the liquid crystal optical element100is tilted to the right inFIG. 2(a) is taken as (+), and the angle by which the liquid crystal optical element100is tilted to the left inFIG. 2(a) is taken as (−).

FIG. 3is a schematic cross-sectional view for explaining the structure of the liquid crystal optical element100.

InFIG. 3, an arrow indicates the direction of the light beam emitted from the light source11. For convenience of explanation, the thicknesses of the respective elements are exaggerated, and their ratio does not represent the actual thickness ratio.

A first transparent electrode151and a first alignment film152are formed on the surface of the first transparent substrate101that faces the second transparent substrate102, and a first transparent counter electrode154and a second alignment film153are formed on the surface of the second transparent substrate102that faces the first transparent substrate101. As earlier described, the first liquid crystal layer110is sealed between the first and second transparent substrates101and102by means of the sealing member130, etc.

A second transparent counter electrode155and a third alignment film156are formed on the surface of the second transparent substrate102that faces the third transparent substrate103, and a second transparent electrode158and a fourth alignment film157are formed on the surface of the third transparent substrate103that faces the second transparent substrate102. As earlier described, the second liquid crystal layer120is sealed between the second and third transparent substrates102and103by means of the sealing member131, etc.

The first and second liquid crystal layers110and120are formed from the same liquid crystal and to the same thickness in view of mass-producibility. Further, the first and second liquid crystal layers110and120are homogeneously aligned (horizontally aligned) liquid crystal layers. The liquid crystal material used for the first and second liquid crystal layers110and120is a positive nematic liquid crystal with a Δn of 0.21 and a relative permittivity of 6.9; the layer thickness is 6.0 μm.

For the first liquid crystal layer110to provide a sufficient amount of phase difference, it is preferable that the refractive index of the liquid crystal material be 0.15 or higher; the higher the refractive index, the better the performance, but a refractive index of 0.3 or less is preferable. This is because a prescribed amount of total phase difference is needed in order for the liquid crystal layers to function as the aberration correcting and nλ/4 plates. Further, if the first and second liquid crystal layers110and120are made too thin, there arises a manufacturing problem, and if they are made too thick, the response speed of the liquid crystal decreases. Therefore, it is preferable that the first and second liquid crystal layers110and120be made not thinner than 3 μm but not thicker than 7 μm.

InFIGS. 2 and 3, the liquid crystal optical element100is constructed by sandwiching the two liquid crystal layers alternately between the three transparent glass substrates. Alternatively, the liquid crystal optical element100may be constructed by forming the liquid crystal layers each between two transparent glass substrates and by bonding them together using an adhesive or the like.

FIG. 4is a diagram showing the relationship between the rubbing direction of each liquid crystal layer and the tilt of the liquid crystal optical element.

FIG. 4(a) shows an arrangement in which the Z-axis of the liquid crystal optical element100is oriented parallel to the optical axis (the optical element is not tilted), andFIG. 4(b) shows an arrangement in which the liquid crystal optical element100is tilted by the angle α as previously shown inFIG. 2.

InFIG. 4(a), reference numeral400indicates the X-Y plane, and401the long axis direction of the liquid crystal molecules in the second liquid crystal layer120that functions as the nλ/4 plate. The angle α is the tilt angle of the liquid crystal optical element100. The angle θ is the pretilt angle of the second liquid crystal layer120. The angle Ψ is the angle that the rubbing direction121of the second liquid crystal layer120makes with the rubbing direction111of the first liquid crystal layer110.

When the liquid crystal optical element100is tilted by the angle α, the pretilt angle θ of the second liquid crystal layer120, relative to the optical axis3, is as shown inFIG. 4(b).

Usually, the angle ψ that the rubbing direction111of the first liquid crystal layer110makes with the rubbing direction121of the second liquid crystal layer120is set to 45 degrees so that the prescribed linearly polarized light passed through the polarization beam splitter13can be converted to nearly perfect circularly polarized light by the second liquid crystal layer120functioning as the nλ/4 plate.

However, when the liquid crystal optical element100is tilted by the angle α in a direction perpendicular to the rubbing direction111of the first liquid crystal layer110in order to prevent reflection, etc., the pretilt angle θ of the second liquid crystal layer120is not canceled by the tilt angle α, as can be seen fromFIG. 4(b). Therefore, the angle ψ that the rubbing direction111of the first liquid crystal layer110makes with the rubbing direction121of the second liquid crystal layer120need not necessarily be set to 45 degrees, but should be set to an angle that can produce a better result. In view of this, in the liquid crystal optical element100of the present invention, the angle ψ is determined as will be described later.

FIG. 5is a diagram for explaining one example of the electrode pattern of the first transparent electrode and its function.

FIG. 5(a) shows a spherical aberration correcting transparent electrode pattern formed as the first transparent electrode151,FIG. 5(b) shows an example of the voltage applied to the transparent electrode pattern of the first transparent electrode151, andFIG. 5(c) shows an example of spherical aberration as improved by the transparent electrode pattern of the first transparent electrode151.

There are case where, due to imperfections such as irregularities in the thickness of the optically transmissive protective layer on the track surface of the recording medium4, the distance between the objective lens14and the track surface varies or the light spot cannot be always focused in the same condition. Such variations in the distance between the objective lens14and the track surface cause spherical aberration in the substrate of the recording medium4, leading to a degradation of the light intensity signal which is generated based on the reflected beam from the recording medium4. One example of the spherical aberration measured at the position of the entrance pupil of the objective lens14is shown by520inFIG. 5(b).

InFIG. 5(a), nine concentric electrode patterns501to509are formed within the range of the effective diameter2. Voltage510such as shown inFIG. 5(b) is applied to each region. When the voltage510such as shown inFIG. 5(b) is applied to the transparent electrode pattern of the first transparent electrode151such as shown inFIG. 5(a), a potential difference occurs with respect to the first transparent counter electrode154, and the alignment of the liquid crystal between them changes according to the potential difference. As a result, the light beam passing through this portion experiences an effect that advances its phase according to the potential difference. With this effect, the spherical aberration520occurring in the substrate of the recording medium4is corrected as shown by the spherical aberration530inFIG. 5(c). Here, the voltage to be applied to the transparent electrode pattern of the first transparent electrode151is supplied via the flexible board140.

In the present embodiment, the first liquid crystal layer110has been constructed to correct spherical aberration but, for aberrations other than spherical aberration, it can be constructed to correct coma or astigmatism, as will be described later.

FIG. 6is a diagram showing one example of the electrode pattern of the second transparent electrode.

A circular transparent electrode pattern550, which is formed as the electrode pattern of the second transparent electrode158, is for generating a potential difference with respect to the second transparent counter electrode155and thereby causing the second liquid crystal layer120to function as an nλ/4 plate. The shape of the transparent electrode pattern550is not limited to a circular shape, the only requirement being that the pattern be sized and shaped so as to contain the effective diameter2of the light beam. The voltage to be applied to the transparent electrode pattern of the second transparent electrode158is supplied via the flexible board140.

FIG. 7is a diagram showing the relationship between the potential difference occurring between the second transparent electrode158and second transparent counter electrode155of the second liquid crystal layer and the amount of phase difference introduced into the light beam passing through the second liquid crystal layer.

FIG. 7shows the results of the measurements made using the light source of 650 nm and the second liquid crystal layer120of 6.0 μm thickness formed from a homogeneously aligned positive nematic liquid crystal (Δn: 0.21, relative permittivity: 6.9). InFIG. 7, curve L1is a graph of the measurements taken when the ambient temperature was 25° C.

As shown by the curve L1, the potential differences where the second liquid crystal layer120functions as a so-called λ/4 plate (that is, as a (½)λ±(n/4)λ plate, where λ=650 nm) are V1where it functions as a 7λ/4 plate, V2where it functions as a 5λ/4 plate, and V3where it functions as a 3λ/4 plate. If the applied voltage to the second liquid crystal layer120is increased, the potential difference where it functions as a λ/4 plate may also be used. However, since the maximum potential difference when the liquid crystal optical element100is used in a small portable apparatus is considered to be about 5 V, the values up to the potential difference of 5V are shown inFIG. 7.

By considering the wavelength of the light source used and the structure of the second liquid crystal layer120in conjunction with the various conditions under which the liquid crystal optical element100is operated, it can be determined as which type of nλ/4 plate the second liquid crystal layer should be made to function.

FIGS. 8 to 13are diagrams showing the relationship between the angle ψ and ellipticity when the tilt angle α is varied from +0.5 degree to +3.0 degrees in increments of 0.5 degree.

In making the measurements, the potential difference to be applied was determined using the curve L1(ambient temperature: 25° C.) shown inFIG. 7, the second liquid crystal layer was made to function as a 7λ/4, 5λ/4, and 3λ/4 plate, respectively, and the ellipticity was measured by varying the angle ψ from 41 degrees to 49 degrees in increments of one degree. The measurement of the ellipticity was performed using a rotating analyzer method at an ambient temperature of 25° C. That is, for the light beam (650 nm) passing through the liquid crystal optical element100, the optical power ratio was measured by an optical power meter while rotating the polarizer. Further, in making the measurements, a special element in which the second liquid crystal layer was formed in such a manner as to be rotatable relative to the first liquid crystal layer was fabricated. Further, the first and second liquid crystal layers were each formed from a homogeneously aligned positive nematic liquid crystal (Δn: 0.21, relative permittivity: 6.9) with a layer thickness of 6.0 μm (the same structure as the second liquid crystal layer inFIG. 7).

Here, for the liquid crystal layer to function effectively as an nλ/4 plate, it is desirable that the ellipticity be not smaller than 0.95.

FIG. 8is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +0.5 degree.

InFIG. 8, curve M10shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M11shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M12shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 8, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 45 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 44 to 46 degrees.

FIG. 9is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +1.0 degree.

InFIG. 9, curve M20shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M21shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M22shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 9, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 45 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 44 to 46 degrees.

FIG. 10is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +1.5 degrees.

InFIG. 10, curve M30shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M31shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M32shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 10, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 46 degrees, and functions effectively as the so-called λ/4 plate when the angle γ is within the range of 45 to 47 degrees.

FIG. 11is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +2.0 degrees.

InFIG. 11, curve M40shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M41shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M42shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 11, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 46 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 45 to 47 degrees.

FIG. 12is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +2.5 degrees.

InFIG. 12, curve M50shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M51shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M52shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 12, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 43 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 42 to 44 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 47 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 46 to 48 degrees.

FIG. 13is a diagram showing the relationship between the angle ψ and the ellipticity when the angle α is +3.0 degrees.

InFIG. 13, curve M60shows the case where the second liquid crystal layer120was made to function as a 7λ/4 plate, curve M61shows the case where the second liquid crystal layer120was made to function as a 5λ/4 plate, and curve M62shows the case where the second liquid crystal layer120was made to function as a 3λ/4 plate. The plus and minus signs of the angle α indicate the same directions as those defined inFIG. 2(a).

As shown inFIG. 13, it can be seen that when the second liquid crystal layer120is made to function as a 7λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 44 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 43 to 45 degrees. It can also been seen that when the second liquid crystal layer120is made to function as a 5λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 43 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 42 to 44 degrees. Further, it can be seen that when the second liquid crystal layer120is made to function as a 3λ/4 plate, the second liquid crystal layer120functions most effectively as the so-called λ/4 plate when the angle ψ is 47 degrees, and functions effectively as the so-called λ/4 plate when the angle ψ is within the range of 46 to 48 degrees.

FIG. 14is a flow diagram illustrating the process for determining the angle ψ.

First, the angle α (seeFIGS. 2 and 4) at which to mount the liquid crystal optical element100in the optical pickup apparatus10is set (S1).

Next, the maximum value of the supply voltage that can be supplied to the liquid crystal optical element100, i.e., the maximum value of the potential difference that can be generated between the second transparent electrode158and second transparent counter electrode155of the second liquid crystal layer, is set (S2).

The angle α and the maximum potential difference are set in advance as the specification for the liquid crystal optical element.

Next, based on the maximum potential difference set in S2, it is determined how the n in the nλ/4 should be set (S3). For example, in the example shown inFIG. 7, when the maximum potential difference is 2.5 [Vrms], it is determined that n=5, i.e., the second liquid crystal layer120should be made to function as a 5λ/4 plate. A plurality of n's may be selected based on the maximum potential difference.

Next, from the n in the nλ/4 determined in S3and the angle α set in S1, the angle ψ is determined (S4) to complete the process. For example, when the angle α is set to +1.5 degrees, it is determined fromFIG. 10that the best angle ψ for achieving the 5λ/4 plate is 44 degrees.

When the liquid crystal optical element100is fabricated by setting the rubbing direction121of the second liquid crystal layer120so as to match the angle ψ (for example, 44 degrees) determined by the process ofFIG. 14, and the thus fabricated liquid crystal optical element100is mounted by tilting it at the angle α (+1.5 degrees) in the optical pickup apparatus10, the second liquid crystal layer120functions effectively as the so-called λ/4 plate.

The examples ofFIGS. 7 to 13have been shown to exemplify the relationship between the potential difference and the ellipticity and the relationship between the angle ψ and the ellipticity for various values of the angle α for the case of the typical light source wavelength and the typical liquid crystal layer. When actually designing the second liquid crystal layer, the optimum angle ψ is determined in accordance with the process shown inFIG. 14, based on the angle α and the maximum potential difference and on the data defining the relationship between the potential difference and the ellipticity and the relationship between the angle ψ and the ellipticity for the liquid crystal layer to be fabricated. The description so far given has dealt with the structure of the liquid crystal optical element100for the case where the ambient temperature is 25° C. However, in the liquid crystal optical element100according to the present invention, if the ambient temperature deviates from 25° C., the angle once set based on the angle α need not be changed, and the liquid crystal layer can be made to function as the intended nλ/4 plate by controlling in accordance with the temperature change the voltages to be applied to the first and second liquid crystal layers110and120.

FIG. 15is a diagram for explaining another example of the electrode pattern of the first transparent electrode and its function.

FIG. 15(a) shows a coma correcting transparent electrode pattern formed as the first transparent electrode151,FIG. 15(b) shows an example of the voltage applied to the transparent electrode pattern of the first transparent electrode151, andFIG. 15(c) shows an example of coma as improved by the transparent electrode pattern of the first transparent electrode151.

In the optical pickup apparatus10which reads or writes data on the recording medium4, as shown inFIG. 1the light beam from the light source11is converted by the collimator lens12into a substantially parallel beam of light, which is then focused by the objective lens14onto the recording medium4, and an information signal is generated by receiving the light beam reflected from the recording medium4. In such an optical pickup apparatus, when reading or writing data on the recording medium, the light beam focused by the objective lens14must be made to accurately follow the track of the recording medium4. However, there are cases where the recording medium4is tilted because of warping or deflection of the recording medium4or imperfections in the driving mechanism of the recording medium4. When the optical axis of the light beam focused by the objective lens14is tilted relative to the track of the recording medium4, an aberration called coma occurs in the substrate of the recording medium4; i.e., coma620such as shown inFIG. 15(b) occurs when seen at the position of the entrance pupil of the objective lens14, and this can lead to a degradation of the information signal which is generated based on the reflected beam from the recording medium4.

InFIG. 15(a), electrode patterns601to605are formed as shown within the range of the effective diameter2. Voltage610such as shown inFIG. 15(b) is applied to each region. When the voltage610such as shown inFIG. 15(b) is applied to the transparent electrode pattern of the first transparent electrode151such as shown inFIG. 15(a), a potential difference occurs with respect to the first transparent counter electrode154, and the alignment of the liquid crystal between them changes according to the potential difference. As a result, the light beam passing through this portion experiences an effect that advances its phase according to the potential difference. With this effect, the coma620occurring in the substrate of the recording medium4is corrected as shown by the coma630inFIG. 15(c). The voltage to be applied to the transparent electrode pattern of the first transparent electrode151is supplied via the flexible board140.

The pattern of the first transparent electrode151shown inFIG. 15(a) described above can be used in place of the transparent electrode pattern shown inFIG. 5. In that case, the liquid crystal optical element100can correct coma.

FIG. 16is a diagram for explaining still another example of the electrode pattern of the first transparent electrode and its function.

FIG. 16(a) shows an astigmatism correcting transparent electrode pattern formed as the first transparent electrode151,FIG. 16(b) shows an example of the voltage applied in the Y-axis direction of the transparent electrode pattern of the first transparent electrode151, andFIG. 16(c) shows an example of astigmatism in the Y-axis direction as improved by the transparent electrode pattern of the first transparent electrode151.FIG. 17(a) shows the transparent electrode pattern ofFIG. 16(a) when it is rotated through 90 degrees,FIG. 17(b) shows an example of the voltage applied in the X-axis direction of the transparent electrode pattern of the first transparent electrode151, andFIG. 17(c) shows an example of astigmatism in the X-axis direction as improved by the transparent electrode pattern of the first transparent electrode151.

In the optical pickup apparatus10which reads or writes data on the recording medium4, due to the problem of the astigmatic difference of the semiconductor laser, etc. astigmatism720such as shown inFIG. 16(b) occurs in the Y-axis direction in the light beam emitted from the light source11, and astigmatism722such as shown inFIG. 17(b) occurs in the X-axis direction, leading to a degradation of the information signal which is generated based on the reflected beam from the recording medium4.

InFIG. 16(a) or17(a), electrode patterns701to709are formed as shown within the range of the effective diameter2. Voltage710such as shown inFIG. 16(b) and voltage712such as shown inFIG. 17(b) are applied to each region. When the voltage710such as shown inFIG. 16(b) and the voltage712such as shown inFIG. 17(b) are applied to the transparent electrode pattern of the first transparent electrode151such as shown inFIG. 16(a) or17(a), a potential difference occurs with respect to the first transparent counter electrode154, and the alignment of the liquid crystal between them changes according to the potential difference. As a result, the light beam passing through this portion experiences an effect that advances its phase according to the potential difference. With this effect, the astigmatism720in the Y-axis direction and the astigmatism722in the X-axis direction, occurring in the substrate of the recording medium4, are corrected as shown by the astigmatism730inFIG. 16(c) and the astigmatism732inFIG. 17(c), respectively. The voltage to be applied to the transparent electrode pattern of the first transparent electrode151is supplied via the flexible board140.

The pattern of the first transparent electrode151shown inFIGS. 16(a) and17(a) described above can be used in place of the transparent electrode pattern shown inFIG. 5. Therefore, the liquid crystal optical element100can correct astigmatism.