JP4695739B2 - Optical element and optical head and optical recording / reproducing apparatus using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical element that changes the phase of incident light, and an optical head and an optical recording / reproducing apparatus using the optical element.
[0002]
[Prior art]
An optical recording medium such as a digital versatile disk (DVD) is attracting attention as a large-capacity optical recording medium because it can record digital information with high density. Here, in order to record / reproduce digital information with high density, it is necessary to shorten the wavelength of light for recording / reproduction and to increase the NA (numerical aperture) of the objective lens. However, when the wavelength of light is shortened and the NA of the lens is increased, the wavefront aberration, particularly coma aberration, increases due to deviation (tilt) from the optical axis caused by warpage of the optical recording medium, and the margin for tilt is smaller. There was a problem of becoming.
[0003]
In order to solve this problem, an optical head that corrects wavefront aberration using a liquid crystal panel has been proposed (Japanese Patent Laid-Open No. 9-128785).
[0004]
An example of the above-described conventional optical head will be described with reference to FIG.
[0005]
FIG. 23 shows a configuration diagram of a conventional optical head 1 (also referred to as an optical pickup). The optical head 1 includes a light source 2, a half mirror 3 a, an objective lens 3 b, a condenser lens 3 c, an optical element 4, a tilt sensor 5, an optical element control circuit 6, and a photodetector 7.
[0006]
The light source 2 is made of, for example, a semiconductor laser element, and is an optical recording medium 8 (a medium for recording information, and a recording medium from which recorded information is read using light. Examples thereof include a CD and a DVD. ) Is output to the recording layer. The optical element 4 includes a liquid crystal panel, and the liquid crystal panel has a plurality of segment electrodes having a pattern as shown in FIG. The optical element 4 changes the refractive index of the liquid crystal for each segment electrode by applying a desired voltage to each segment electrode, and changes the phase of light transmitted through each segment electrode. Therefore, the optical element 4 can correct the aberration of light incident on the optical element 4.
[0007]
The function of the conventional optical head 1 will be described. The linearly polarized light emitted from the light source 2 is reflected by the half mirror 3 a and enters the optical element 4. Here, if the optical recording medium 8 is tilted from the perpendicular to the optical axis, a signal corresponding to the tilt amount (tilt angle) is output by the tilt sensor 5. Based on the signal output from the tilt sensor 5, the optical element control circuit 6 causes the liquid crystal of the optical element 4 to generate a phase change necessary for correcting the wavefront aberration that occurs when the optical recording medium 8 is tilted. Control the panel. Thus, the light incident on the optical element 4 is given a phase change that corrects the wavefront aberration that occurs when the recording medium 8 is tilted. The light transmitted through the optical element 4 is condensed on the optical recording medium 8 by the objective lens 3b. Here, since the light to which the phase change that corrects the wavefront aberration generated when the optical recording medium 8 is tilted is condensed by the objective lens 3b, a light spot (without aberration) on the optical recording medium 8 is collected. A light spot that is narrowed to the diffraction limit) is formed. Next, the light reflected by the optical recording medium 8 becomes light having wavefront aberration corresponding to the inclination of the optical recording medium 8, but the wavefront aberration is corrected by the optical element 4. The light that has passed through the optical element 4 passes through the half mirror 3a, enters the condenser lens 3c without returning to the light source 1, and is condensed on the photodetector 7 by the condenser lens 3c. The photodetector 7 outputs information recorded on the optical recording medium 8. The light detector 7 outputs a focus error signal indicating the focused state of light on the optical recording medium 8 and outputs a tracking error signal indicating the light irradiation position.
[0008]
Here, the principle of tilt correction using the optical element 4 will be described.
[0009]
An example of wavefront aberration distribution at the best image point of the optical recording medium 8 (the tilt angle of the optical recording medium 8 is 1 °, the NA of the objective lens is 0.6, the wavelength is 655 nm, and the substrate thickness of the optical recording medium 8 is 0. FIG. 25 shows the case of 6 mm. As shown in FIG. 25, when the optical recording medium 8 is tilted, the wavefront aberration has a left-right antisymmetrical and substantially semicircular distribution. By applying a phase change that cancels the wavefront aberration distribution of FIG. 25 to the incident light using the optical element 4, the spot on the optical recording medium 8 is narrowed to the diffraction limit even when the optical recording medium 8 is tilted. Can do. In addition, by applying a phase change that cancels the wavefront aberration to the light reflected by the optical recording medium 8, the light detection by the photodetector 7 can be performed with high accuracy.
[0010]
In order to give the incident light a phase change that cancels the wavefront aberration shown in FIG. 25, the optical path length in the optical element 4 may be partially changed. Here, since the refractive index of the liquid crystal changes according to a voltage applied from the outside, the optical path length can be partially changed by partially changing the applied voltage. Therefore, the wavefront aberration shown in FIG. 25 can be corrected by applying externally different voltages for each segment electrode to the segment electrodes having a finely divided pattern as shown in FIG.
[0011]
However, in the optical element 4, it is necessary to externally supply a control signal corresponding to each segment electrode of the liquid crystal panel in the optical element 4. That is, a flexible substrate having the same number of lines as the segment electrodes of the liquid crystal panel from the liquid crystal panel drive circuit must be connected to the optical element 4. Therefore, in the case of the optical element 4 having a large number of segment electrodes as shown in FIG. 24, it is necessary to supply a large number of signals, and the width of the flexible substrate is increased accordingly. When such a wide flexible substrate is connected to the optical element 4, it is very difficult to adjust the components, and further, the optical head 1 is greatly reduced in size. Further, it is very difficult to bond a large number of lines to the optical element 4 which is a small optical component without short-circuiting, and as the number of lines increases, the yield of the bonding process of the flexible substrate to the optical element 4 increases. It becomes worse and the cost of the optical head 1 increases.
[0012]
In order to solve these problems, an optical element having a segment electrode different from the conventional segment electrode has been proposed (Japanese Patent Laid-Open No. 10-20263). The electrode shape of this segment electrode is shown in FIG. This segment electrode has a shape corresponding to the shape of the wavefront aberration that occurs when the optical recording medium 8 is tilted. Therefore, compared with the conventional optical element, the wavefront aberration can be considerably corrected even if the number of segment electrodes is reduced.
[0013]
[Problems to be solved by the invention]
However, even in the optical element having the segment electrode having the shape shown in FIG. 26, the pattern shown in FIG. 26 needs to be divided more finely in order to correct the wavefront aberration more accurately. Therefore, also in the optical element having the segment electrode having the shape shown in FIG. 26, the number of control signals increases according to the degree of division of the segment electrode, and the connection between the optical element 4 and the optical element control circuit 6 becomes difficult. There is a problem. Furthermore, there is a problem that it is difficult to downsize the optical head.
[0014]
In addition, on the portion (separation portion) between the segment electrode and the segment electrode, there is a problem that the influence of the electric field is weak and correction of wavefront aberration is not sufficient. Here, it is conceivable to reduce the width of the separation part. However, if the width of the separation part is made narrower than a certain value, there arises a problem that the manufacturing is difficult and the yield is lowered.
[0015]
In order to solve the above problems, an object of the present invention is to provide an optical element that has a high correction effect on incident light and is easy to manufacture, and an optical head and an optical recording / reproducing apparatus using the optical element.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, an optical element of the present invention includes a voltage application electrode including a plurality of segment electrodes, a counter electrode disposed substantially parallel to the voltage application electrode so as to face the voltage application electrode, A phase change layer made of a phase change material disposed between the voltage application electrode and the counter electrode, wherein the voltage application electrode further includes a voltage control electrode made of a conductive substance, and the voltage control electrode is external The voltage applied from is divided by the resistance of the conductive material and applied to the plurality of segment electrodes, and the voltage difference between the voltage application electrode and the counter electrode is changed to thereby change the phase change layer. The voltage control electrode is disposed at a position that does not overlap the phase change layer. In addition, the plurality of segment electrodes include a plurality of semicircular electrodes arranged substantially symmetrically. It is characterized by that. According to the optical element, an optical element that has a high correction effect on incident light and is easy to manufacture can be obtained.
[0017]
In the optical element, it is preferable that the phase change material is a material whose refractive index changes due to the voltage difference. According to the above configuration, the phase of incident light can be easily changed.
[0018]
In the optical element, the phase change material is preferably a liquid crystal. According to the above configuration, the voltage applied to change the phase of the incident light can be small.
[0019]
In the optical element, it is preferable that the phase change material is a material whose volume changes due to the voltage difference. According to the above configuration, the phase of incident light can be easily changed.
[0020]
In the optical element, the phase change material is preferably PLZT. According to the said structure, an element can be made thin.
[0022]
In the optical element, it is preferable that the plurality of segment electrodes include a plurality of substantially semicircular electrodes arranged substantially symmetrically. According to the above configuration, wavefront aberration can be corrected easily and accurately.
[0023]
In the optical element, it is preferable that the plurality of segment electrodes include a plurality of electrodes divided concentrically. According to the above configuration, spherical aberration can be corrected easily and accurately.
[0024]
In the optical element, the thickness d of the voltage application electrode _a But d _a = (2N _a +1) λ / 2n _a (Where λ is the wavelength of the incident light, N _a Is an integer greater than or equal to 0, n _a Is preferably a thickness represented by the refractive index of the voltage application electrode). According to the above configuration, when the refractive indexes of the upper and lower layers of the voltage application electrode are substantially equal, reflection of light at the voltage application electrode can be prevented.
[0025]
In the optical element, the thickness d of the counter electrode _b But d _b = (2N _b +1) λ / 2n _b (Where λ is the wavelength of the incident light, N _b Is an integer greater than or equal to 0, n _b Is preferably a thickness represented by the refractive index of the counter electrode). According to the said structure, when the refractive index of the upper and lower layers of a counter electrode is substantially equal, reflection of the light by a counter electrode can be prevented.
[0026]
The optical element preferably further includes an antireflection film for preventing reflection of incident light. According to the above configuration, loss of light due to reflection can be prevented.
[0027]
In the optical element, the voltage application electrode is divided into the plurality of segment electrodes by a separation unit, and the width of the separation unit is such that the entire region of the phase change layer located on the separation unit is affected by the segment electrode. It is preferable that the width be received. According to the above configuration, the phase of the light that has passed through the portion where the segment electrode is not formed can be controlled, so that an optical element having a particularly high phase correction effect can be obtained.
[0028]
In the optical element, it is preferable that the width W of the separation portion and the thickness d of the phase change layer satisfy a relationship of W ≦ 3d. According to the above configuration, an optical element having a higher phase correction effect can be obtained.
[0029]
In the optical element, it is preferable that the voltage application electrode is divided into the plurality of segment electrodes by a separation part, and further includes a light shielding film that shields light passing through the separation part. According to the above configuration, since light that has passed through the portion of the liquid crystal that is not controlled by the electric field can be cut, an optical element having a particularly high phase correction effect is obtained.
[0030]
In the optical element, the light shielding film is preferably made of metal. With the above configuration, a light shielding film having a high light shielding effect can be easily formed.
[0031]
The optical head of the present invention is an optical head that reads information recorded on an optical recording medium by light, and includes a light source, and an optical element disposed between the optical recording medium and the light source, The optical element is the optical element of the present invention. Since the optical head includes the optical element of the present invention, an optical head that has a high correction effect on incident light and is easy to manufacture can be obtained.
[0032]
The optical head preferably further includes an N / 4 wavelength plate (where N is an odd number of 1 or more) disposed between the optical recording medium and the optical element. With the above configuration, the light utilization efficiency is increased, and signal recording can be performed.
[0033]
The optical recording / reproducing apparatus of the present invention is an optical recording / reproducing apparatus for recording or reproducing signals (including when recording and reproducing) on an optical recording medium, and for recording signals on the optical recording medium. Alternatively, the optical head includes a reproducing optical head, and the optical head includes a light source and an optical element disposed between the optical recording medium and the light source, and the optical element is the optical element of the present invention. It is characterized by. Since the optical recording / reproducing apparatus includes the optical element of the present invention, an optical recording / reproducing apparatus having a high correction effect on incident light and easy to manufacture can be obtained.
[0034]
The optical recording / reproducing apparatus preferably further includes an N / 4 wavelength plate (where N is an odd number of 1 or more) disposed between the optical recording medium and the optical element.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0036]
(Embodiment 1)
In Embodiment 1, an example of the optical element of the present invention will be described.
[0037]
A perspective view of the optical element 10 of Embodiment 1 is shown in FIG. 1, and a cross-sectional view of the optical element 10 is shown in FIG.
[0038]
Referring to FIGS. 1 and 2, an optical element 10 includes a first substrate 11, a second substrate 12 disposed substantially parallel to the first substrate 11, a first substrate 11, and a liquid crystal 17. A voltage applying electrode 13 disposed between the electrodes, a counter electrode 14 disposed substantially parallel to the voltage applying electrode so as to face the voltage applying electrode 13, and a translucency formed so as to cover the voltage applying electrode 13 The resin film 15, the translucent resin film 16 formed so as to cover the counter electrode 14, and the translucent resin films 15 and 16 (between the voltage application electrode 13 and the counter electrode 14) are disposed. The liquid crystal 17 and the sealing resin 18 arrange | positioned between the translucent resin films 15 and 16 so that the liquid crystal 17 may be enclosed are included.
[0039]
The first substrate 11 and the second substrate 12 are made of glass, for example, and are translucent.
[0040]
The voltage application electrode 13 is an electrode for applying a desired voltage to the liquid crystal 17. The voltage application electrode 13 is formed on the main surface on the inner side (liquid crystal 17 side) of the first substrate 11.
[0041]
The counter electrode 14 is an electrode for applying a desired voltage to the liquid crystal 17 together with the voltage application electrode 13. The counter electrode 14 is formed on the main surface on the inner side (the liquid crystal 17 side) of the second substrate 12. The counter electrode 14 is connected to an electrode 19 formed on the first substrate 11 through a conductive resin (not shown) formed on a part of the sealing resin 18. The electrode 19 is connected to, for example, the ground (GND). The counter electrode 14 is translucent and is made of, for example, ITO. The counter electrode 14 is formed substantially uniformly on at least a portion of the inner main surface of the second substrate 12 facing the segment electrode portion 13a.
[0042]
The translucent resin films 15 and 16 are alignment films for aligning the liquid crystal 17 in a predetermined direction, and are made of, for example, a polyvinyl alcohol film. By rubbing the translucent resin film 15 or 16, the liquid crystal 17 can be aligned in a predetermined direction. The liquid crystal 17 may be aligned by other methods (for example, oblique vapor deposition).
[0043]
The liquid crystal 17 functions as a phase change layer that changes the phase of incident light. The liquid crystal 17 is made of, for example, nematic liquid crystal. By changing the voltage difference between the voltage application electrode 13 and the counter electrode 14, the refractive index of the liquid crystal 17 can be changed, whereby the phase of incident light can be changed. Instead of the liquid crystal 17, another phase change layer made of a phase change material may be used (the same applies to the following embodiments). As the phase change material, a material whose refractive index and volume (film thickness) change according to the voltage difference between the voltage application electrode 13 and the counter electrode 14 can be used. An example of a material whose refractive index changes with a voltage difference is liquid crystal. Examples of the material whose volume changes with a voltage difference include PLZT (perovskite structure transparent crystal containing lead oxide, lanthanum, zirconium oxide, and titanium oxide). Here, when a solid such as PLZT is used for the phase change layer, either the voltage application electrode 13 or the counter electrode 14 may be formed on the phase change layer. Therefore, in this case, either the first substrate 11 or the second substrate 12 may be omitted. Therefore, when PLZT is used, the element can be thinned.
[0044]
The sealing resin 18 is for sealing the liquid crystal 17 and is made of, for example, an epoxy resin.
[0045]
FIG. 3 shows a plan view of the voltage application electrode 13. Referring to FIG. 3, voltage application electrode 13 includes a segment electrode portion 13a and a voltage control electrode 13b.
[0046]
As shown in FIG. 3, the segment electrode portion 13a includes a plurality of segment electrodes A, B, C, D, and E. The segment electrodes A to E and the voltage control electrode 13b are electrically connected by lead lines La to Le. The segment electrodes A and E are substantially semicircular. The combined shape of segment electrode A and segment electrode B is substantially semicircular, and segment electrodes D and E are the same. Further, the combined shape of the segment electrodes A to E is a substantially perfect circle. The segment electrodes A and B and the segment electrodes D and E are arranged substantially symmetrically on the segment electrode C. The shape of the segment electrode changes according to the aberration distribution of light to be corrected, and the shape shown in FIG. 3 is an example.
[0047]
The segment electrode part 13a consists of a translucent electrode, for example, ITO can be used. The voltage control electrode 13b is made of a conductive material. In addition, the voltage application electrode 13 can be easily manufactured by using the same material for the segment electrode portion 13a and the voltage control electrode 13b.
[0048]
The voltage control electrode 13b controls the voltage applied from the outside and applies it to the segment electrode part 13a. Voltages V1 and V2 (external signals V1 and V2) are applied from outside to predetermined positions of the voltage control electrode 13b. FIG. 4 shows an equivalent circuit of the voltage control electrode 13b. Here, r1 is a resistance between the lead lines La and Lb, r2 is a resistance between the lead lines Lb and Lc, r3 is a resistance between the lead lines Lc and Ld, and r4 is a resistance between the lead lines Ld and Le. It is. The voltages V1 and V2 applied to the voltage control electrode 13b are divided by the resistance of the voltage control electrode 13b, and the voltages Va, Vb, Vc, Vd and Ve are output to the lead lines La to Le. The output voltages Va to Ve can be freely changed according to the material, shape, and the like of the voltage control electrode 13b. Since the liquid crystal 17 functions as a very small capacitor, a signal for driving the liquid crystal 17 (for example, a rectangular wave having a frequency of 1 kHz) hardly flows to the liquid crystal 17. Therefore, the voltages Va to Ve output from the voltage control electrode 13b are applied to the segment electrode portion 13a with almost no change.
[0049]
Here, FIG. 5 shows the relationship between the control voltages Va to Ve applied to the liquid crystal 17 (effective value when the applied voltage is a rectangular wave of 1 kHz, for example) and the refractive index of the liquid crystal 17 with respect thereto. As shown in FIG. 5, the refractive index of the liquid crystal 17 hardly changes until the control voltage reaches a certain level, decreases linearly when the control voltage exceeds a certain threshold value, and then further exceeds a certain threshold value. Almost no change. Therefore, by controlling the liquid crystal 17 with a voltage at which the refractive index changes almost linearly with respect to the control voltage, the phase change of the light transmitted through the segment electrode can be changed linearly with the applied voltage. it can.
[0050]
In the optical element 10, the refractive index of the liquid crystal 17 changes and the optical path length changes according to the magnitude of the voltage difference generated between the segment electrode portion 13 a to which the voltages Va to Ve are applied and the counter electrode 14. . Therefore, a desired phase change can be given to the light transmitted through the liquid crystal 17 by changing the voltages Va to Ve, and for example, wavefront aberration generated due to the tilt of the optical recording medium can be corrected.
[0051]
For example, in order to correct the wavefront aberration (see FIG. 25) that occurs when the tilt is 1 °, the phase change of the light transmitted through the segment electrodes A to E is changed to the segment electrode A, the segment electrode B, the segment electrode C, and the segment. What is necessary is just to make it large in order of the electrode D and the segment electrode E. Further, the wavefront aberration of FIG. 25 is transmitted through each segment electrode with reference to the phase change of the light transmitted through the segment electrode C because the absolute values of the left and right phase differences with respect to the center are substantially equal and opposite in sign. What is necessary is just to make it the phase change of light become substantially antisymmetric on right and left. That is, the difference between the phase change of the light transmitted through the segment electrode A and the phase change of the light transmitted through the segment electrode C is the difference between the phase change of the light transmitted through the segment electrode E and the phase change of the light transmitted through the segment electrode C. It is preferable that the absolute value is equal to the difference and the sign is opposite. Similarly, the difference between the phase change of light transmitted through the segment electrode B and the phase change of light transmitted through the segment electrode C is the same as the phase change of light transmitted through the segment electrode D and the phase change of light transmitted through the segment electrode C. It is preferable that the absolute value is equal to the difference between and the sign is opposite.
[0052]
In order to change the phase of the light transmitted through the segment electrodes as described above, the voltages Va to Ve applied to the segment electrodes A to E are Va = Vc + X, Vb = Vc + Y, Vd = Vc−Y, Ve = Vc. It is preferable to satisfy the relationship of -X (where X and Y are both positive values, and X is larger than Y). In order to establish the above relationship, r1 and r4 should be made equal and r2 and r3 should be made equal in the equivalent circuit shown in FIG.
[0053]
As an example, let us consider a case where ITO is used for the voltage application electrode 13 and r1, r2, r3 and r4 are all set to 1 KΩ. For example, 1mm ² When an ITO film having a resistance value of about 30Ω is used, the distance between adjacent lead lines may be set to 100 μm and the width of the voltage control electrode 13b may be set to 3.3 μm. By changing the value of V1 or V2 in this situation, the applied voltages Va to Ve applied to the segment electrodes A to E can be changed while satisfying the above relationship.
[0054]
The thickness d of the segment electrode portion 13a _a D _a = (2N _a +1) λ / 2n _a (Where λ is the wavelength of the incident light, N _a Is an integer greater than or equal to 0, n _a Is preferably a thickness represented by the refractive index of the voltage application electrode. The segment electrode portion 13a is sandwiched between the first substrate 11 (refractive index is approximately 1.5) and the translucent resin film 15 (refractive index is approximately 1.5). Therefore, when ITO having a refractive index of 2, for example, is used for the segment electrode part 13a, the segment electrode part 13a is equivalent to the absence of the segment electrode part 13a by satisfying the above formula, and the segment electrode part 13a Can prevent reflection. When the refractive indexes of the upper and lower layers of the voltage application electrode 13 are different, the denominator of the above formula is 4n. _a It is preferable that
[0055]
Further, for the same reason as the thickness of the segment electrode portion 13a, the thickness d of the counter electrode 14 _b D _b = (2N _b +1) λ / 2n _b (Where λ is the wavelength of the incident light, N _b Is an integer greater than or equal to 0, n _b Is preferably a thickness expressed by the refractive index of the counter electrode). When the refractive indexes of the upper and lower layers of the counter electrode 14 are different, the denominator of the above formula is 4n. _b It is preferable that
[0056]
Next, an example of a method for manufacturing the optical element 10 will be described with reference to FIGS.
[0057]
When manufacturing the optical element 10, first, the voltage application electrode 13 and the electrode 19 are formed on the first substrate 11. The voltage application electrode 13 and the electrode 19 can be formed by forming a light-transmitting conductive film such as ITO by sputtering, for example, and then patterning by a photolithography process and an etching process. Thereafter, the translucent resin film 15 is formed by, for example, spin coating so as to cover the voltage application electrode 13 and the electrode 19. On the other hand, the counter electrode 14 and the translucent resin film 16 are formed on the second substrate 12 in parallel with the above process. The counter electrode 14 and the translucent resin film 16 can be formed by the same method as the voltage application electrode 13 and the translucent resin film 15. Thereafter, the first substrate 11 and the second substrate 12 that have undergone the above steps are opposed to each other with the sealing resin 18 interposed therebetween, and the liquid crystal 17 is sealed between the translucent resin films 15 and 16. In this way, the optical element 10 can be formed.
[0058]
As described above, in the optical element 10 of the first embodiment, the voltages V1 and V2 applied to the voltage control electrode 13b are divided into voltages Va to Ve by the resistance of the voltage control electrode 13b, and each segment of the segment electrode portion 13a. Applied to electrodes A-E. For this reason, in the optical element 10, a high correction effect is obtained with respect to the incident light, and the number of signal lines connected to the optical element 10 is small. Therefore, according to the optical element 10, an optical element that has a high correction effect on incident light and is easy to manufacture can be obtained.
[0059]
In the first embodiment, the voltage control electrode 13b having the shape of FIG. 3 is shown as an example of the voltage control electrode 13b. However, the shape of the voltage control electrode 13b is not limited to the shape of FIG. For example, the voltage control electrode 13b may have a shape as shown in FIG. 6 or FIG. 7 according to required resistance values r1 to r4. In the voltage control electrode shown in FIG. 6A, the width between the lead lines La and Lb and between Ld and Le is narrower than the width between the lead lines Lb and Ld. In the voltage control electrode shown in FIG. 6B, the length between the lead lines La-Lb and between Ld-Le is longer than the length between the lead lines Lb-Lc and between Lc-Ld. The voltage control electrode shown in FIG. 7 has a bent shape. By making the voltage control electrode 13b into the shape shown in FIG. 6 (a) or (b), r1 and r4 can be made larger than r2 and r3. Moreover, the value of r1-r4 can be arbitrarily changed by making the voltage control electrode 13b into the shape shown in FIG.
[0060]
Moreover, in the optical element 10 of Embodiment 1, the case where the segment electrode part 13a of the voltage application electrode 13 and the voltage control electrode 13b were comprised with the same material was shown, However, The voltage control electrode 13b is shown as needed. A material different from the material of the segment electrode part 13a may be used as the material. Further, the voltage control electrode 13b may be composed of a plurality of conductive substances.
[0061]
FIG. 8 shows a case where the material of the voltage control electrode is different from the material of the segment electrode. Referring to FIG. 8, voltage application electrode 80 (corresponding to voltage application electrode 13 in FIG. 3) includes segment electrode portion 80a and voltage control electrode 80b.
[0062]
The segment electrode part 80a is a translucent electrode, and for example, ITO can be used.
[0063]
The voltage control electrode 80b is formed of a material different from that of the segment electrode portion 80a, and for example, Ge, Ti, or W can be used as necessary. When a high-resistance material is used for the voltage control electrode 80b, since the current flowing through the voltage control electrode 80b can be reduced, the burden on the IC for driving the liquid crystal can be reduced, and a highly reliable optical element Is obtained. Further, when a high resistance material is used for the voltage control electrode 80b, a desired resistance value can be obtained without reducing the width of the voltage control electrode 80b, so that the optical element can be easily manufactured.
[0064]
In the first embodiment, the segment electrode portion 13a is divided into the segment electrodes A to E. However, the aberration can be corrected more accurately by further dividing the segment electrode portion 13a (hereinafter referred to as the segment electrode portion 13a). This is the same in the embodiment). For example, aberration can be corrected more accurately by using a segment electrode portion 13a having a three-stage pattern (consisting of seven segment electrodes) as shown in FIG. For example, considering the case of correcting the wavefront aberration (see FIG. 25) when the optical recording medium is tilted by 1 °, the wavefront aberration at the best image point is 80 mλ without correction, but one-step pattern ( When the segment electrode portion 13a having the pattern of FIG. 26 and including three segment electrodes) is used, the wavefront aberration is reduced to 60 mλ by the correction. Further, when correction is performed using the segment electrode portion 13a having a two-stage pattern (the pattern of FIG. 3 and consisting of five segment electrodes), the three-stage pattern (the pattern of FIG. The wavefront aberration is reduced to 40 mλ. In the optical element 10 of the first embodiment, even if the number of divisions of the segment electrode portion 13a is increased, the external voltages input to the optical element 10 are only V1, V2, and ground, and do not increase.
[0065]
In the first embodiment, the optical element in the case of correcting the wavefront aberration caused by the tilt of the optical recording medium has been described. However, the spherical aberration can also be corrected by changing the shape of the voltage application electrode. When correcting the spherical aberration, segment electrodes divided concentrically as shown in FIG. 10 may be used (the same applies to the following embodiments).
[0066]
(Embodiment 2)
In Embodiment 2, another example of the optical element of the present invention will be described.
[0067]
A sectional view of the optical element 110 according to the second embodiment is shown in FIG.
[0068]
Referring to FIG. 11, the optical element 110 includes a first substrate 11, a second substrate 12, a voltage application electrode 13, a counter electrode 14, and the optical element 10 described in the first embodiment. Translucent resin films 15 and 16, a liquid crystal 17, and a sealing resin 18 are provided. Further, the optical element 110 is disposed between the antireflection film 111 formed on the main surface outside the first substrate 11 (opposite the liquid crystal 17), and between the first substrate 11 and the voltage application electrode 13. The interlayer antireflection film 112, the interlayer antireflection film 113 disposed between the voltage application electrode 13 and the translucent resin film 15, and the counter electrode 14 and the translucent resin film 16 are disposed. Interlayer antireflection film 114, interlayer antireflection film 115 disposed between second substrate 12 and translucent resin film 16, and main outside of second substrate 12 (on the opposite side to liquid crystal 17) And an antireflection film 116 formed on the surface.
[0069]
The first substrate 11, the second substrate 12, the voltage application electrode 13, the counter electrode 14, the translucent resin films 15 and 16, the liquid crystal 17, the sealing resin 18, and the electrode 19 are those described in the first embodiment. Since it is the same as that, the overlapping description is omitted.
[0070]
The antireflection films 111 and 116 and the interlayer antireflection films 112 to 115 are formed to prevent reflection of light transmitted through the optical element 110. The antireflection films 111 and 116 and the interlayer antireflection films 112 to 115 can be formed by, for example, a sputtering method or a vapor deposition method.
[0071]
Here, a general antireflection film will be described with reference to FIG. As shown in FIG. 12, the refractive index is n. ₁ ~ N _Three In the case where light is transmitted through the mediums 1 to 3, the film thickness L of the medium 2 is L = (2N ₂ +1) λ / 4n ₂ (Where λ is the wavelength of transmitted light, N ₂ Is an integer greater than or equal to 0, n ₂ Is the refractive index of the medium 2, the reflectance at the medium 2 is minimized. Further, the refractive index n of the medium 2 ₂ Is the refractive index n of the medium 1 ₁ And refractive index n of medium 3 _Three And the geometric mean (n ₁ And n _Three The reflectance at the medium 2 can be further reduced.
[0072]
Therefore, it is preferable to use a film satisfying the relationship between the film thickness and the refractive index as the antireflection films 111 and 116 and the interlayer antireflection films 112 to 115.
[0073]
Here, considering the refractive indexes of the antireflection films 111 and 116, when the refractive index of air is 1, and the refractive indexes of the first substrate 11 and the second substrate 12 are 1.5, for example (for example, When normal glass is used for the first substrate 11 and the second substrate 12), the refractive index of the antireflection film is preferably close to 1.22. Accordingly, magnesium fluoride (refractive index 1.38) having a refractive index close to 1.22 can be used for the antireflection films 111 and 116.
[0074]
Next, considering the refractive indexes of the interlayer antireflection films 112 and 115, for example, the refractive indexes of the first substrate 11 and the second substrate 12 are 1.5, and the refraction of the voltage application electrode 13 and the counter electrode 14 is considered. When the rate is 2 (for example, when ITO is used for the voltage application electrode 13 and the counter electrode 14), the refractive indexes of the interlayer antireflection films 112 and 115 are preferably close to 1.73. Therefore, alumina (refractive index of 1.68) having a refractive index close to 1.73 can be used for the interlayer antireflection films 112 and 115.
[0075]
Similarly, considering the refractive indexes of the interlayer antireflection films 113 and 114, for example, the refractive index of the voltage application electrode 13 and the counter electrode 14 is 2, and the refractive indexes of the translucent resin films 15 and 16 are 1.5. In the case of translucent resin films 15 and 16 (for example, polyvinyl alcohol films), it is preferable that the refractive indexes of interlayer antireflection films 113 and 114 are close to 1.73. Therefore, the interlayer antireflection films 113 and 114 can be made of alumina or the like having a refractive index close to 1.73. In addition, since the refractive index of the translucent resin films 15 and 16 and the refractive index of the liquid crystal 17 are substantially equal, it is not necessary to consider the reflection between the translucent resin films 15 and 16 and the liquid crystal 17.
[0076]
Note that the refractive index and material of the antireflection film are merely examples, and it goes without saying that the optimum refractive index and film thickness for the antireflection film vary depending on the material used for the optical element 110 and the wavelength of transmitted light. Absent.
[0077]
Further, the antireflection films 111 and 116 and the interlayer antireflection films 112 to 115 may not be a single layer, but may be a laminate of a plurality of thin films having different refractive indexes. By using an antireflection film composed of a plurality of thin films, the reflectance can be further reduced.
[0078]
In the optical element 110 of the second embodiment, the same effect as that of the optical element 10 of the first embodiment can be obtained. Further, in the optical element 110, since the antireflection films 111 and 116 and the interlayer antireflection films 112 to 115 are formed, the incident light is attenuated by reflection on the surface and inside of the optical element 110. Can be prevented.
[0079]
(Embodiment 3)
In Embodiment 3, another example of the optical element of the present invention will be described.
[0080]
A sectional view of the optical element 130 of Embodiment 3 is shown in FIG.
[0081]
Referring to FIG. 13, the optical element 130 includes a first substrate 11, a second substrate 12 disposed substantially parallel to the first substrate 11, and between the first substrate 11 and the liquid crystal 17. The arranged voltage application electrode 131, the counter electrode 14 arranged substantially parallel to the voltage application electrode so as to face the voltage application electrode 131, and the translucent resin film 15 formed so as to cover the voltage application electrode 131 A translucent resin film 16 formed so as to cover the counter electrode 14, and a liquid crystal 17 disposed between the translucent resin films 15 and 16 (between the voltage application electrode 131 and the counter electrode 14). And a sealing resin 18 disposed between the translucent resin films 15 and 16 so as to surround the liquid crystal 17.
[0082]
In addition, since it is the same as that of the optical element 10 of Embodiment 1 about the part except the voltage application electrode 131, overlapping description is abbreviate | omitted.
[0083]
A partial plan view of the voltage application electrode 131 is shown in FIG. FIG. 14 also shows the alignment direction of the liquid crystal, the polarization direction of incident light, and the radial direction of the optical recording medium. In FIG. 14, the display of the extraction electrode is omitted, but as the extraction electrode, an electrode as shown in FIG. 3 can be used.
[0084]
The voltage application electrode 131 is an electrode for applying a desired voltage to the liquid crystal 17 and thereby controlling the phase of light passing through the liquid crystal 17. The voltage application electrode 131 is divided into a plurality of segment electrodes 133 a to 133 e by the separation unit 132. The segment electrodes 133a to 133e are translucent electrodes, and for example, ITO can be used. The shape of the segment electrode is preferably changed according to the aberration distribution of light to be corrected, and the shape shown in FIG. 14 is an example.
[0085]
The width W of the separation portion 132 is a width that is affected by the segment electrode in the entire area of the liquid crystal (phase change layer) 17 located on the separation portion 132. That is, the width W of the separation part 132 is a distance equal to or smaller than the width where the liquid crystal 17 affected by a certain segment electrode and the liquid crystal 17 affected by a segment adjacent to the segment electrode are in contact. Specifically, when the thickness of the liquid crystal 17 in the direction perpendicular to the first substrate 11 is d (see FIG. 13), the width W and the thickness d satisfy the relationship of W ≦ 3d or less. It is particularly preferable that the relationship of W ≦ 2d is satisfied. The width W is, for example, not less than 1 μm and not more than 15 μm.
[0086]
Next, the behavior of the liquid crystal 17 on the separation unit 132 will be described in detail. Usually, at the ends of the two electrodes facing each other, the electric field is spread and distributed in a space without electrodes. Such a state is schematically shown in FIG. In the case of a parallel plate capacitor having two semi-infinite electrodes facing each other, the relationship between the distance from the end of the electrode (distance in the X-axis direction in FIG. 15) and its potential (insulation effect of the parallel plate capacitor) is It can be clarified by using an isometric map (see “Electromagnetism”, published by Asakura Shoten, written by Miharu Fujimoto, “Practical electromagnetism”, published by Kyoritsu Publishing, Kenichi Goto, and Shuichiro Yamazaki). Here, since the magnitude of the electric field is a differential coefficient of the electric potential, by differentiating the electric potential in the direction between the electrodes (the Y-axis direction in FIG. 15), in the two semi-infinite parallel plate capacitors facing each other, The relationship between the distance from the end and the electric field in the direction perpendicular to the electrode can be obtained. The results calculated in this way are shown in FIG. In FIG. 16, the horizontal axis represents the distance in the X-axis direction from the electrode end, and is a value normalized with the distance between two opposing electrodes as 1. In addition, 0 on the horizontal axis is the end of the electrode, and the greater the value of x, the farther away from the electrode. On the other hand, the Y-axis indicates the electric field strength in the Y-axis direction when the electric field strength between the electrodes is 100.
[0087]
From FIG. 16, the electric field leaks to the portion where there is no electrode, and when it is separated by the same distance as the distance between the electrodes (x = 1), the electric field is almost 20% of the electric field between the electrodes (y = 100). It can be seen that Accordingly, even in an actual optical element, the refractive index of the liquid crystal positioned on the separation portion 132 is changed by this electric field. In addition, since the segment electrode is on both sides of the separation part 132, the liquid crystal directly under the separation part 132 is affected by the electric field leaking from the adjacent segment electrode. Here, FIG. 16 shows the calculation result when the two electrodes are both semi-infinite, but in an actual optical element, the segment electrode is semi-infinite and the counter electrode must be considered as infinite. . In this case, the electric field lines do not go around to the back side of the infinite electrode (opposite electrode), so the electric field strength at a certain distance from the semi-infinite electrode is considered to be larger than that shown in FIG. It is done. Therefore, it is considered that the electric field at a place (x = 1) separated by the distance between the electrodes leaks considerably from the end portions of the electrodes.
[0088]
Actually, an optical element including the voltage application electrode 131 shown in FIG. 14 was made as a prototype by setting the thickness d of the liquid crystal to 5 μm and the width W of the separation portion 132 to 10 μm (2d). Aberration correction was performed with an optical head using this optical element (see FIG. 18). When the optical recording medium is tilted by 1 °, the jitter is 20% or more when the aberration is not corrected, but when the aberration is corrected using the optical element, the jitter is 7.5%. Here, since the jitter when the optical recording medium is not tilted was 6.6%, it is considered that the coma caused by the tilting of the optical recording medium was almost completely corrected by the optical element. Therefore, it is considered that the liquid crystal on the separation portion 132 has a substantially optimum refractive index due to the electric field leaking from the adjacent segment electrode.
[0089]
Here, from FIG. 16, the magnitude of the electric field at the position of x = 1.5 (when the width of the separation portion is three times the thickness d of the liquid crystal because the separation portion is adjacent to the segment) is x = It is about 2/3 of the position of 1 (the width W of the separating portion is twice the thickness d of the liquid crystal). Considering that the aberration correction was performed almost completely when the width W of the separation portion was twice the thickness d of the liquid crystal, even if the width W of the separation portion was increased to three times the thickness of the liquid crystal d. It is considered that the aberration can be sufficiently corrected by the electric field leaking onto the separation part.
[0090]
Next, the light transmittance of the optical element will be examined. When the same voltage is applied to all the segment electrodes, the refractive index of the liquid crystal in the portion affected by the segment electrode is different from the refractive index of the liquid crystal in the portion not affected by the segment electrode. For this reason, a part of the incident light is diffracted and the light transmittance of the optical element is lowered. Even in such a case, according to the optical element of the above embodiment, since all of the liquid crystal positioned on the segment electrode portion is affected by the segment electrode, the decrease in light transmittance is small. Actually, for the optical element 130, the light transmittance was measured by setting all the segment electrodes to 3V and the counter electrode to 0V. As a result, when the thickness d of the liquid crystal was 5 μm and the width W of the separation portion was 10 μm, the light transmittance of the optical element was 91%. When the thickness d of the liquid crystal was 5 μm and the width W of the separation part was 5 μm, the light transmittance of the optical element was 97%. Although the light transmittance depends on the area of the separation part, it cannot be generally stated. However, from the above results, it is preferable that the thickness d of the liquid crystal and the width W of the separation part satisfy the relationship of W ≦ d. all right.
[0091]
As described above, in the optical element 130, the refractive index of the liquid crystal 17 can be changed in each region by controlling the voltage difference between the segment electrodes 133 a to 133 e and the counter electrode 14. Therefore, in the optical element 130, the optical path length at a desired position of the liquid crystal 17 can be changed. Here, as shown in FIG. 14, the polarization direction of the light incident on the optical element 130, the alignment direction of the liquid crystal 17, and the radial direction of the optical recording medium are substantially parallel. Therefore, even when the phase distribution (coma aberration) as shown in FIG. 25 occurs due to the tilt of the optical recording medium, the voltage applied to each segment electrode 133a-e is controlled as shown in FIG. The coma aberration can be corrected by providing the incident light with a phase distribution having a polarity opposite to the phase distribution.
[0092]
In particular, in the optical element 130, the width W of the separation portion 132 is a width that is affected by the segment electrode in the entire area of the liquid crystal (phase change layer) 17 positioned on the separation portion 132, and thus the liquid crystal 17 on the separation portion 132. The phase of the light that has passed through is also controlled. Therefore, according to the optical element 130, an optical element having a particularly high correction effect on incident light can be obtained. Further, in the optical element 130, a sufficient correction effect can be obtained without making the width of the separating portion 132 unnecessarily narrow, so that an optical element that can be manufactured with high yield and low cost can be obtained.
[0093]
Note that the shape of the voltage application electrode 131 shown in FIG. 14 is an example, and electrodes having other shapes can be used as long as the width W of the separation portion is not more than a predetermined value.
[0094]
Further, the voltage control electrode described in Embodiment 1 or the antireflection film described in Embodiment 2 may be formed on the optical element 130.
[0095]
(Embodiment 4)
In Embodiment 4, another example of the optical element of the present invention will be described.
[0096]
A cross-sectional view of the optical element 170 of Embodiment 4 is shown in FIG.
[0097]
Referring to FIG. 17A, the optical element 170 includes a first substrate 11, a second substrate 12 disposed substantially parallel to the first substrate 11, the first substrate 11, and the liquid crystal 17. The voltage application electrode 171 disposed between the electrode, the counter electrode 14 disposed substantially parallel to the voltage application electrode 171 so as to face the voltage application electrode 171, and a translucent formed so as to cover the voltage application electrode 171 The transparent resin film 15, the translucent resin film 16 formed so as to cover the counter electrode 14, and the translucent resin films 15 and 16 (between the voltage application electrode 171 and the counter electrode 14) are disposed. Liquid crystal 17, sealing resin 18 disposed between translucent resin films 15 and 16 so as to surround liquid crystal 17, and main surface on the outer side of first substrate 11 (the side on which liquid crystal 17 does not exist) And a light shielding film 172 formed on the substrate.
[0098]
Since the portion excluding the voltage application electrode 171 and the light shielding film 172 is the same as that of the optical element 10 of the first embodiment, a duplicate description is omitted.
[0099]
The voltage application electrode 171 is an electrode for applying a desired voltage to the liquid crystal 17 and thereby controlling the phase of light passing through the liquid crystal 17. The voltage application electrode 171 is divided into a plurality of segment electrodes by the separation part, similarly to the voltage application electrode 131 of FIG. 14, but the voltage application electrode 171 may have a wider separation part. Each segment electrode is a translucent electrode, and for example, ITO can be used.
[0100]
The light shielding film 172 is a film that shields part of incident light. For the light shielding film 172, for example, a metal can be used. Specifically, for example, an aluminum film having a thickness of 100 nm can be used. FIG. 17A shows the case where the light shielding film 172 is formed on the first substrate 11, but the light shielding film 172 may be formed on the second substrate 12. The light shielding film 172 can be formed, for example, by depositing a metal thin film and then removing unnecessary portions of the metal in a photolithography process and an etching process.
[0101]
An example plan view of the light shielding film 172 is shown in FIG. The light shielding film 172 shown in FIG. 17B is a case where the voltage application electrode 171 has a pattern similar to the voltage application electrode 131 in FIG. 14 (the width of the separation portion is different).
[0102]
Referring to FIG. 17B, the light shielding film 172 is formed at a position corresponding to the separation portion. In other words, the light shielding film 172 is provided on the optical path of the light that passes through the separation portion among the light incident on the optical element 170.
[0103]
As described in the third embodiment, when the width of the separation portion of the voltage application electrode is wide, the liquid crystal 17 on the separation portion is not sufficiently controlled, so that the phase of light transmitted through this portion is sufficiently controlled. Therefore, the aberration correction becomes insufficient. On the other hand, in the optical element 170, light having an uncontrolled phase is shielded by the light shielding film 172, so that only light having an optimal phase is transmitted through the optical element 170 and sufficient aberration correction is performed. be able to.
[0104]
Actually, in the optical element 170 including the voltage applying electrode 171 and the light shielding film 172 having a wide separation portion, the jitter was 8% when the aberration was corrected when the tilt was 1 °, and a good result was obtained. . On the other hand, when aberration correction was not performed in the same case, the jitter was 20%. Thus, it has been found that the optical element 170 can provide a high aberration correction effect.
[0105]
As described above, in the optical element 170 according to the fourth embodiment, an optical element having an excellent aberration correction effect can be obtained by shielding all the light passing through the separation portion of the voltage application electrode 171. In the optical element 170, since the width of the separation portion can be increased, the optical element can be easily manufactured at a low cost and with a high yield.
[0106]
The light shielding film 172 may shield all of the light passing through the segment electrode separation portion, or may shield only a part of the light. That is, since the liquid crystal 17 on the separation part is affected by the electric field leaking from the segment electrode as described in the third embodiment, the aberration correction is sufficiently performed by shielding only the light passing through the part not affected by the electric field. It can be carried out. When only a part of the light passing through the separation unit is shielded, the amount of light that can be used increases, so that jitter can be further improved.
[0107]
Note that the shape of the light shielding film 172 illustrated in FIG. 17B is an example, and the shape of the light shielding film 172 changes according to the shape of the separation portion of the segment electrode.
[0108]
Further, the voltage control electrode described in Embodiment 1 or the antireflection film described in Embodiment 2 may be formed on the optical element 170.
[0109]
(Embodiment 5)
In the fifth embodiment, an optical head using the optical element of the present invention described in any of the first to fourth embodiments will be described.
[0110]
The configuration of the optical head 180 of Embodiment 5 is schematically shown in FIG.
[0111]
Referring to FIG. 18, the optical head 180 includes a light source 181, a diffraction grating 182, a collimator lens 183, an optical element 184, an objective lens 185, a tilt sensor 186, and photodetectors 187 and 188. .
[0112]
The light source 181 includes, for example, a semiconductor laser element, and emits recording / reproducing coherent light to the recording layer of the optical recording medium 189.
[0113]
The diffraction grating 182 is a grating formed by etching a photoresist having a desired pattern on the glass surface using photolithography. The diffraction grating 182 has a zero-order diffraction efficiency of approximately 50% and a ± first-order diffraction efficiency of approximately 50%. That is, the diffraction grating 182 functions as a separating unit that separates the reflected light from the optical recording medium 189 from the optical path of the light emitted from the light source 181.
[0114]
The collimator lens 183 and the objective lens 185 constitute a condensing optical system.
[0115]
As the optical element 184, the optical element of the present invention described in any of Embodiments 1 to 4 is used. That is, the optical element 184 is an element that corrects aberration by partially changing the refractive index of the liquid crystal by applying different voltages to the segment electrodes of the voltage application electrode.
[0116]
The objective lens 185 is a lens that condenses light on the recording layer of the optical recording medium 189.
[0117]
The tilt sensor 186 detects the tilt angle of the optical recording medium 189. The tilt sensor 186 outputs a signal corresponding to the tilt angle of the optical recording medium 189 to the optical element control circuit 190.
[0118]
The optical element control circuit 190 is a circuit that applies electrical signals (for example, V1 and V2 in FIG. 3) to the optical element 184 in accordance with the signal output from the tilt sensor 186.
[0119]
The photodetector 187 receives + first-order light diffracted by the diffraction grating 182 among the light reflected by the recording layer of the optical recording medium 189 and converts it into an electrical signal.
[0120]
The photodetector 188 receives -first-order light diffracted by the diffraction grating 182 among the light reflected by the recording layer of the optical recording medium 189 and converts it into an electrical signal.
[0121]
The function of the optical head 180 will be described with reference to FIG. Part of the linearly polarized light emitted from the light source 181 passes through the diffraction grating 182 and enters the collimator lens 183. The light incident on the collimator lens 183 is converted into substantially parallel light by the collimator lens 183 and is incident on the optical element 184.
[0122]
Here, when the optical recording medium 189 is tilted from the perpendicular to the optical axis, a signal for correcting wavefront aberration according to the tilt amount (tilt angle) is output from the tilt sensor 186, and the signal is optical. Input to the element control circuit 190. The optical element control circuit 190 outputs a signal necessary for correcting wavefront aberration caused by tilt to the optical element 184. Accordingly, the light incident on the optical element 184 is given a phase change that corrects the wavefront aberration caused by the tilt of the optical recording medium 189.
[0123]
The light transmitted through the optical element 184 is collected on the optical recording medium 189 by the objective lens 185. The light focused on the optical recording medium 189 is given a phase change that corrects the wavefront aberration by the optical element 184, so that there is no aberration on the optical recording medium 189, that is, the diffraction limit is reduced. A light spot is formed.
[0124]
The light reflected by the optical recording medium 189 becomes light having wavefront aberration corresponding to the tilt of the optical recording medium 189, but the wavefront aberration is corrected again by the optical element 184.
[0125]
The light reflected by the optical recording medium 189 and transmitted through the optical element 184 passes through the collimator lens 183 and is diffracted by the diffraction grating 182. Of the diffracted light diffracted by the diffraction grating 182, the + 1st order light is incident on the photodetector 187 and the −1st order light is incident on the photodetector 188.
[0126]
The photodetector 187 outputs a focus error signal indicating the focused state of light on the optical recording medium 189 and a tracking error signal indicating the light irradiation position. The focus error signal is output to focus control means (not shown). Based on the focus error signal, the focus control means controls the position of the objective lens 185 in the optical axis direction so that the light is always focused on the optical recording medium 189 in a focused state. The tracking error signal is input to tracking control means (not shown). The tracking control means controls the position of the objective lens 185 based on the tracking error signal so that the light is condensed on a desired track on the optical recording medium 189.
[0127]
The photodetector 188 detects the record information recorded on the optical recording medium 189.
[0128]
Next, the optical element control circuit 190 when the optical element 184 is the optical element described in Embodiment 1 or 2 will be described. A circuit diagram of the optical element control circuit 190 in this case is shown in FIG. The optical element control circuit 190 includes signal sources 191 and 192, an operational amplifier 193, a delay circuit 194, and a switch 195.
[0129]
The signal sources 191 and 192 output an electrical signal to the voltage control electrode of the optical element 184. The electrical signals output by the signal sources 191 and 192 change according to the voltages V1 and V2 applied to the voltage control electrodes. The operational amplifier 193 has a variable gain. The delay circuit 194 adjusts the phase of the signal from the signal source 192 so that the phase of V1 and the phase of V2 are in phase. The switch 195 distributes the electrical signal output from the signal sources 191 and 192 to the voltage V1 or V2 applied to the voltage control electrode. The signal source 191 or 192 corresponding to the voltage V1 or V2 can be switched by the switch 195.
[0130]
When correcting the wavefront aberration caused by the tilt of the optical recording medium 189, if the tilt direction of the optical recording medium 189 is the same and changes by the magnitude, the wavefront aberration can be corrected by changing the gain of the operational amplifier 193. Also, when the direction of warping of the optical recording medium 189 is reversed, the signal source 191 or 192 corresponding to V1 and V2 is switched by the switch 195, and the gain of the operational amplifier 193 is changed as necessary. Wavefront aberration can be corrected.
[0131]
Here, in the optical head 180, FIG. 20 shows the measurement results of jitter when the wavefront aberration is corrected using the optical element 10 of Embodiment 1 and when the wavefront aberration is not corrected. As apparent from FIG. 20, the jitter margin can be increased by correcting the wavefront aberration using the optical element 184.
[0132]
As described above, since the optical head 180 according to the fifth embodiment includes the optical element of the present invention, an optical head that can read signals recorded on the optical recording medium with high reliability is obtained. Further, in the optical head 180, since the jitter margin or the tilt margin is increased by using the optical element of the present invention, an optical head that is easy to manufacture and low in cost can be obtained.
[0133]
Further, when the optical element according to the first or second embodiment is used, the number of signals applied to the optical element 184 from the outside can be reduced, so that an optical head that can correct aberrations and is small and easy to manufacture can be obtained. can get.
[0134]
(Embodiment 6)
In Embodiment 6, another example of the optical head of the present invention will be described.
[0135]
The configuration of the optical head 180a of Embodiment 6 is schematically shown in FIG.
[0136]
Referring to FIG. 21, an optical head 180a includes a light source 181, a collimator lens 183, an optical element 184, an objective lens 185, a tilt sensor 186, photodetectors 187 and 188, a polarization hologram 211, 1 / 4 wavelength plate 212. The polarization hologram 211 is disposed between the light source 181 and the collimator lens 183. The quarter wavelength plate 212 is disposed between the optical element 184 and the optical recording medium 189.
[0137]
Since the portion excluding the polarization hologram 211 and the quarter wavelength plate 212 is the same as that described in the fifth embodiment, a duplicate description is omitted. The optical element 184 is the optical element described in any of Embodiments 1 to 4.
[0138]
The polarization hologram 211 is an optical element that transmits an extraordinary ray as it is and functions as a diffraction grating for the ordinary ray. The polarization hologram 211 can be formed, for example, by exchanging a part of a lithium niobate substrate having birefringence and etching the proton exchange part (see JP-A-6-27322).
[0139]
The quarter-wave plate 212 converts linearly polarized light emitted from the light source 181 into circularly polarized light, and reflects light reflected by the recording layer of the optical recording medium 139 in a direction different from the linearly polarized light. It is a nonlinear optical element that converts to polarized light. The quarter wave plate 212 is made of, for example, quartz. Instead of the quarter wavelength plate, an N / 4 wavelength plate (N is an odd number of 3 or more) may be used.
[0140]
The operation of the optical head 180a will be described with reference to FIG. The linearly polarized light emitted from the light source 181 passes through the polarization hologram 211 as it is, enters the collimator lens 183, becomes parallel light by the collimator lens 183, and enters the optical element 184. Here, when the optical recording medium 189 is tilted from the perpendicular to the optical axis, a signal corresponding to the tilt amount (tilt angle) is output from the tilt sensor 186, and the output signal is input to the optical element control circuit 190. Is done. Based on the output signal, the optical element control circuit 190 outputs to the optical element 184 a signal necessary for correcting wavefront aberration that occurs when the optical recording medium 189 is tilted. In this way, the phase of the light input to the optical element 184 is controlled so as to correct the wavefront aberration caused by the tilt of the optical recording medium 189.
[0141]
The light transmitted through the optical element 184 is incident on the quarter-wave plate 212, and its polarization state is converted from linearly polarized light to circularly polarized light. This circularly polarized light is condensed on the optical recording medium 189 by the objective lens 185 and reflected. Here, the light having wavefront aberration for correcting the wavefront aberration generated when the optical recording medium 189 is tilted is collected by the objective lens 185, so that there is no aberration on the optical recording medium 189, that is, the diffraction limit. A light spot is formed.
[0142]
The light reflected by the optical recording medium 189 passes through the objective lens 185 and enters the quarter wavelength plate 212. The light incident on the quarter wavelength plate 212 is converted from circularly polarized light to linearly polarized light by the quarter wavelength plate 212. The linearly polarized light that has passed through the quarter-wave plate 212 becomes linearly polarized light that is orthogonal to the linearly polarized light emitted from the light source 181. This linearly polarized light passes through the optical element 184 and the collimator lens 183 and is diffracted by almost 100% by the polarization hologram 211. The diffracted + 1st order light is incident on the photodetector 187, and the diffracted −1st order light is incident on the photodetector 188.
[0143]
The functions of the light detector 187, the light detector 188, and the focus control means (not shown) are the same as those described in the fifth embodiment, and a duplicate description is omitted.
[0144]
As described above, when the polarization optical system is used, the utilization efficiency of the light emitted from the light source 181 increases, and it becomes easy to record and reproduce signals on a rewritable optical recording medium.
[0145]
Next, the position of the optical element 184 that corrects aberration will be described. Since the liquid crystal 17 of the optical element has uniaxial birefringence, as shown in FIG. 14, in order to correct aberration, linearly polarized light substantially parallel to the alignment direction of the liquid crystal 17 must be incident on the optical element 184. When the optical element 184 is disposed between the quarter-wave plate 212 and the optical recording medium 189, circularly polarized light is incident on the optical element 184 having the liquid crystal 17 acting as a birefringent plate. Therefore, the polarization state after passing through the optical element 184 takes an arbitrary state from linearly polarized light to circularly polarized light according to the retardation of the liquid crystal 17. As a result, in the worst case, the light reflected by the optical recording medium 189 enters the polarization hologram 211 in the same polarization state as the light emitted from the light source 181. In this case, since no light enters the photodetectors 187 and 188, the information signal recorded on the optical recording medium 189 cannot be reproduced. Therefore, the optical element 184 needs to be disposed between the light source 181 and the quarter-wave plate 212. In other words, the quarter-wave plate 212 needs to be disposed between the optical element 184 and the optical recording medium 189.
[0146]
In the optical head 180a of the sixth embodiment, the same effect as the optical head 180 described in the fifth embodiment can be obtained. Furthermore, the optical head 180a can increase the light utilization efficiency by arranging the quarter wavelength plate 212 between the optical element 184 and the optical recording medium 189, and can record and reproduce the rewritable optical recording medium. Becomes easy.
[0147]
(Embodiment 7)
In the seventh embodiment, an example of the optical recording / reproducing apparatus of the present invention will be described. The optical recording / reproducing apparatus according to the seventh embodiment is an apparatus that performs signal recording or reproduction (recording and reproduction may be performed) on an optical recording medium.
[0148]
FIG. 22 schematically shows the configuration of the optical recording / reproducing apparatus 220 of the seventh embodiment. The optical recording / reproducing apparatus 220 includes an optical head 180, an optical element control circuit 190, a motor 221, and a processing circuit 222. The optical head 180 has been described in the fifth embodiment, and includes the optical element 184 of the present invention described in any of the first to fourth embodiments. Note that an optical head 180 a may be used instead of the optical head 180.
[0149]
Since the optical head 180 and the optical element control circuit 190 are the same as those described in the fifth embodiment, a duplicate description is omitted.
[0150]
Next, the operation of the optical recording / reproducing apparatus 220 will be described. First, when the optical recording medium 189 is set in the optical recording / reproducing apparatus 220, the processing circuit 222 outputs a signal for rotating the motor 221 to rotate the motor 221. Next, the processing circuit 222 drives the light source 181 to emit light. Light emitted from the light source 181 is reflected by the optical recording medium 189 and enters the photodetectors 187 and 188. The photodetector 187 outputs a focus error signal indicating the focused state of light on the optical recording medium 189 and a tracking error signal indicating the light irradiation position to the processing circuit 222. Based on these signals, the processing circuit 222 outputs a signal for controlling the objective lens 185, thereby condensing the light emitted from the light source 181 onto a desired track on the optical recording medium 189. Further, the processing circuit 222 reproduces information recorded on the optical recording medium 189 based on the signal output from the photodetector 188.
[0151]
Next, control when the optical recording medium 189 is tilted will be described. When the optical recording medium 189 is tilted, a signal corresponding to the tilt of the optical recording medium 189 is output to the processing circuit 222 by the tilt sensor 186. The processing circuit 222 drives the optical element control circuit 190 according to the input signal, whereby a control signal necessary for correcting coma aberration caused by the tilt of the optical recording medium 189 is transmitted to the optical element control circuit 190. Is output to the optical element 184 (for details, see Embodiment 5 or 6). In this way, even if the optical recording medium 189 is tilted, the information signal recorded on the optical recording medium 189 is correctly reproduced.
[0152]
In the optical recording / reproducing apparatus 220 of the seventh embodiment, coma aberration caused by the tilt of the optical recording medium is corrected by the optical element of the present invention. Therefore, according to the optical recording / reproducing apparatus 220, an optical recording / reproducing apparatus that can reliably reproduce an information signal recorded on the optical recording medium can be obtained. Further, since the tolerance for the tilt of the optical recording medium 189 is increased by using the optical element of the present invention, an optical recording / reproducing apparatus that is inexpensive and easy to manufacture can be obtained.
[0153]
In the above embodiment, the reproduction of the information signal is described. However, even when the information signal is recorded, the optical element is controlled in the same manner as the reproduction, so that the information signal can be recorded with high reliability.
[0154]
The embodiments of the present invention have been described above by way of examples. However, the present invention is not limited to the above embodiments, and can be applied to other embodiments based on the technical idea of the present invention.
[0155]
For example, the radial tilt correction has been described in the above embodiment, but the tangential tilt correction can be performed by rotating the segment electrode pattern by 90 degrees. Furthermore, by using two types of optical elements, both radial tilt and tangential tilt can be corrected.
[0156]
Moreover, although the case where the optical element of the present invention is disposed in the parallel system between the collimator lens and the objective lens has been described in the above embodiment, it may be disposed in the diverging system between the light source and the collimator lens. Good.
[0157]
In the above embodiment, an infinite optical head is shown. However, a finite optical head that does not use a collimator lens may be used.
[0158]
In the above embodiment, the case where the wavefront aberration is corrected using the tilt amount detected by the tilt sensor when reproducing the information recorded on the optical recording medium has been described. However, the present invention is not limited to this, and the relationship between the track position and the tilt amount may be learned in advance before reproduction, and the wavefront aberration at each track position may be corrected from the learned tilt amount.
[0159]
In the above embodiment, the reflected light from the optical recording medium is separated from the optical path from the light source by a diffraction grating or the like and is incident on the photodetector. However, the optical path from the light source using an optical element such as a half mirror is used. And may be incident on the photodetector.
[0160]
In the above embodiment, the optical recording medium that records information only by light has been described. However, the same effect can be obtained by using the optical element of the present invention for a magneto-optical recording medium that records information by light and magnetism. Needless to say.
[0161]
In the above embodiment, the case where the optical recording medium is an optical disk has been described. However, the present invention can be applied to an optical information recording / reproducing apparatus that realizes a similar function, such as a card-like optical recording medium.
[0162]
In the optical element of the first or second embodiment, a transistor may be connected to the voltage control electrode to boost the voltage from the outside. Further, a phase delay circuit may be connected to the voltage control electrode to change the phase of the voltage applied to the voltage application electrode.
[0163]
【The invention's effect】
As described above, according to the optical element, the optical head, or the optical recording / reproducing apparatus of the present invention, an optical element, an optical head, or an optical recording / reproducing apparatus that has a high correction effect on incident light and can be easily manufactured can be obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an example of an optical element of the present invention.
FIG. 2 is a cross-sectional view showing an example of the optical element of the present invention.
FIG. 3 is a plan view showing an example of a voltage application electrode for the optical element of the present invention.
FIG. 4 is a circuit diagram showing an equivalent circuit of a voltage control electrode for the optical element of the present invention.
FIG. 5 is a graph showing the relationship between the refractive index of liquid crystal and the control voltage for the optical element of the present invention.
FIG. 6 is a plan view showing another example of the voltage control electrode in the optical element of the present invention.
FIG. 7 is a plan view showing another example of the voltage control electrode in the optical element of the present invention.
FIG. 8 is a plan view showing another example of voltage application electrodes for the optical element of the present invention.
FIG. 9 is a plan view showing an example of a segment electrode for the optical element of the present invention.
FIG. 10 is a plan view showing another example of the segment electrode for the optical element of the present invention.
FIG. 11 is a cross-sectional view showing another example of the optical element of the present invention.
FIG. 12 is a schematic diagram illustrating the function of an antireflection film in the optical element of the present invention.
FIG. 13 is a cross-sectional view showing another example of the optical element of the present invention.
FIG. 14 is a plan view showing an example of a voltage application electrode for the optical element of the present invention.
FIG. 15 is a diagram for explaining the function of the optical element of the present invention.
FIG. 16 is a graph for explaining the function of the optical element of the present invention.
FIG. 17 is a cross-sectional view showing still another example of the optical element of the present invention.
FIG. 18 is a schematic diagram showing an example of the optical head of the present invention.
FIG. 19 is a circuit diagram showing an optical element control circuit used in the optical head of the present invention.
FIG. 20 is a graph showing the effect of aberration correction on the optical head of the present invention.
FIG. 21 is a schematic view showing another example of the optical head of the present invention.
FIG. 22 is a schematic diagram showing an example of the optical recording / reproducing apparatus of the present invention.
FIG. 23 is a schematic diagram showing an example of a conventional optical head.
FIG. 24 is a schematic diagram showing an example of a segment electrode for a conventional optical element.
FIG. 25 is a graph showing an example of wavefront aberration when the tilt angle is 1 °.
FIG. 26 is a schematic diagram showing an example of a voltage application electrode for a conventional optical element.
[Explanation of symbols]
10, 110, 130, 170 Optical element
11 First substrate
12 Second substrate
13, 80, 131, 171 Voltage application electrode
13a, 80a Segment electrode part
13b, 80b Voltage control electrode
14 Counter electrode
17 Liquid crystal (phase change layer)
111, 116 Antireflection film
112, 113, 114, 115 Interlayer antireflection film
132 Separation unit
172 Light-shielding film
180, 180a Optical head
181 Light source
184 Optical element
186 Tilt sensor
189 Optical recording medium
212 1/4 wave plate
220 Optical recording / reproducing apparatus
221 motor
222 Processing circuit
A, B, C, D, E, 133a-e Segment electrode
V1, V2, Va, Vb, Vc, Vd, Ve voltage
d Thickness of liquid crystal
W Separator width

Claims (17)

A voltage application electrode comprising a plurality of segment electrodes;
A counter electrode disposed substantially parallel to the voltage application electrode so as to face the voltage application electrode;
Including a phase change layer made of a phase change material disposed between the voltage application electrode and the counter electrode,
The voltage application electrode further includes a voltage control electrode made of a conductive material,
The voltage control electrode divides a voltage applied from the outside by a resistance of the conductive material and applies the divided voltage to the segment electrodes,
An optical element that changes a phase of light incident on the phase change layer by changing a voltage difference between the voltage application electrode and the counter electrode,
The voltage control electrode is disposed at a position that does not overlap with the phase change layer, and the plurality of segment electrodes include a plurality of semicircular electrodes disposed substantially symmetrically. . A voltage application electrode comprising a plurality of segment electrodes;
A counter electrode disposed substantially parallel to the voltage application electrode so as to face the voltage application electrode;
Including a phase change layer made of a phase change material disposed between the voltage application electrode and the counter electrode,
The voltage application electrode further includes a voltage control electrode made of a conductive material,
The voltage control electrode divides a voltage applied from the outside by a resistance of the conductive material and applies the divided voltage to the segment electrodes,
An optical element that changes a phase of light incident on the phase change layer by changing a voltage difference between the voltage application electrode and the counter electrode,
The voltage control electrode is disposed at a position not overlapping with the phase change layer, and the plurality of segment electrodes include a plurality of concentrically divided electrodes . Said phase change material, the optical element according to claim 1 or 2 which is a material whose refractive index changes by the voltage difference. The optical element according to claim 3 , wherein the phase change material is a liquid crystal. It said phase change material, the optical element according to claim 1 or 2 volume by the voltage difference is a material that changes. The optical element according to claim 5 , wherein the phase change material is PLZT.   The thickness da of the voltage application electrode is represented by da = (2Na + 1) λ / 2na (where λ is the wavelength of incident light, Na is an integer greater than or equal to 0, and na is the refractive index of the voltage application electrode). The optical element according to claim 1. The thickness db of the counter electrode is a thickness represented by db = (2Nb + 1) λ / 2nb (where λ is the wavelength of incident light, Nb is an integer of 0 or more, and nb is the refractive index of the counter electrode). the optical element according to any one of certain claims 1 to 6. The optical element according to any one of claims 1 further comprising an anti-reflection film that prevents reflection of incident light 6. The voltage application electrode is divided into the plurality of segment electrodes by a separation unit,
The width of the separation portion, the optical element according to any one of the to entire area of the phase change layer located on the separation portion claims 1 to width affected by the segment electrode 6. The optical element according to claim 10 , wherein a width W of the separation portion and a thickness d of the phase change layer satisfy a relationship of W ≦ 3d. The voltage application electrode is divided into the plurality of segment electrodes by a separation unit,
The optical element according to any one of claims 1 to 6, further comprising a light shielding film for blocking light passing through the separation unit. The optical element according to claim 12 , wherein the light shielding film is made of metal. An optical head for reading information recorded on an optical recording medium by light,
A light source, and an optical element disposed between the optical recording medium and the light source,
The optical element, an optical head, characterized in that an optical element according to any one of claims 1 to 13. The optical head according to claim 14 , further comprising an N / 4 wavelength plate (where N is an odd number equal to or greater than 1) disposed between the optical recording medium and the optical element. An optical recording / reproducing apparatus for recording or reproducing a signal to / from an optical recording medium,
An optical head for recording or reproducing a signal on the optical recording medium;
The optical head includes a light source, and an optical element disposed between the optical recording medium and the light source,
The optical element, the optical recording and reproducing apparatus, characterized in that an optical element according to any one of claims 1 to 13. The optical recording / reproducing apparatus according to claim 16 , further comprising an N / 4 wavelength plate (where N is an odd number equal to or greater than 1) disposed between the optical recording medium and the optical element.

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