JP2006048886A - Wavefront aberration compensation element, optical pickup, and optical disk apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compensate large wavefront aberration by facilitating manufacture. <P>SOLUTION: A wavelength selection phase plate 12 with thickness in a transmitting plane different in each area reduces a large wavefront aberration by changing a phase distribution of transmitting wavefronts only to a beam of a certain specific wavelength. A liquid crystal layer 10 provided as a cell 10a in each area reduces a residual aberration to be a wavefront aberration compensation element 7 with a wide aberration compensation range. Since the number of steps in a thickness direction of the transmitting plane in the wavelength selection phase plate 12 can be reduced by compensating the residual aberration by the liquid crystal layer 10 provided as a cell 10a, it is possible to make it easy to manufacture the wavefront aberration compensation element 7. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wavefront aberration correction element, an optical pickup, and an optical disc apparatus for dealing with light sources having a plurality of wavelengths.

  In an optical disc apparatus, information is recorded by irradiating a recording surface on which a spiral or concentric track of the optical disc is formed with laser light, and information is recorded / reproduced / erased based on reflected light from the recording surface. It is operating.

  In general, the optical system of the optical pickup schematically includes a light source 101 and an objective lens 102 as shown in FIG. 9, and the objective lens 102 is used to control a minute spot with respect to the information recording medium 103. A lens actuator 104 for focus / track control is provided. Reference numeral 105 denotes detection separation means (beam splitter) for separating reflected light from the information recording medium 103 from incident light and guiding it to the light receiving element 106.

In recent years, DVD (Digital Versatile Disc) whose recording capacity is dramatically larger than CD (Compact Disc) and a disc standard for recording and reproducing using blue light earlier have been proposed as types of optical discs. For discs that are recorded and reproduced using blue light, two standards have been proposed, with substrate thicknesses of 0.1 mm (Blu-ray) and 0.6 mm (AOD). A light source with a wavelength of 785 nm is used for recording and reproduction with respect to a CD, and a light source with a wavelength of 660 nm is used for recording and reproduction with respect to a DVD. On the other hand, a light source having a wavelength of 405 nm is used for recording and reproduction. Table 1 shows the relationship between the disk substrate thickness, the light source wavelength, and the NA (numerical aperture) of the objective lens.

  By the way, a normal objective lens is designed so as to cancel the wavefront aberration generated by the substrate thickness of the disk. For this reason, optical pickups with different substrate thicknesses / wavelengths generate wavefront aberrations and cannot be recorded / reproduced / erased correctly.

  For example, when DVD parallel light is incident on an objective lens that meets the Blu-ray standard with a substrate thickness of 0.1 mm, a wavefront aberration as shown in FIG. 10 occurs. In general, in an optical pickup, in order to reduce the spot diameter of light collected by an objective lens, it is necessary to suppress the wavefront aberration to 0.07λ or less in terms of rms (Marechal diffraction limit). However, the wavefront aberration shown in FIG. 10 has a large wavefront aberration of 0.7 λ in rms, and the spot diameter cannot be reduced with such an optical pickup, and recording / reproducing / erasing cannot be performed correctly.

  In order to deal with this problem, the following three conventional techniques are available as means for recording / reproducing / erasing information even on disks having different substrate thicknesses by selectively correcting the wavefront.

  First, in Patent Document 1, a wavelength selection phase plate is used. In other words, one optical system that outputs and receives laser beams of different wavelengths and the other optical system are provided, and the light emitted from the semiconductor laser of each optical system is combined and the reflected light from the optical recording medium is separated. Optical multiplexing / demultiplexing means (interference filter) that guides to one of the photodetectors is provided, and the wavelength of one and the other from the first and second semiconductor lasers is provided between the interference filter and the objective lens. A wavelength-selective phase plate having a property of changing the phase distribution of the transmitted wavefront with respect to the other wavelength is interposed. As a result, an optical head device capable of reproducing two types of discs having different substrate thicknesses, obtaining good S / N and jitter during reproduction, and obtaining sufficient output and peak intensity during recording. Is provided.

  In Patent Document 2, a liquid crystal element that corrects wavefront aberration is used. In other words, an imaging optical system provided with an aberration correction mechanism is an optical system that obtains a correction signal based on the detected amount of aberration and controls the aberration correction mechanism based on that amount. A divided liquid crystal element is used, and the liquid crystal panel is driven in accordance with the detected aberration pattern. This makes it possible to correct the wavefront aberration of the imaging optical system.

  Furthermore, according to Patent Document 3, spherical aberration correction by divergent light is performed. That is, in an optical head having light sources with different wavelengths, light is incident on the objective lens in a divergent manner by reducing the distance between the light source with a long wavelength and the objective lens, and negative spherical aberration is generated. Since the objective lens is designed so that the spherical aberration becomes small with respect to a light source having a short wavelength, positive spherical aberration is generated when light having a long wavelength is incident. By canceling each other with positive and negative spherical aberration, an optical head with small spherical aberration is realized for both short-wavelength and long-wavelength light sources.

JP 10-334504 A Japanese Patent No. 02895150 Japanese Patent No. 03236203

  For example, the problem of the prior art when correcting the wavefront aberration of a DVD using an objective lens designed in accordance with the Blu-ray standard with a substrate thickness of 0.1 mm will be described.

  First, the case of the wavelength selection phase plate of Patent Document 1 will be described. FIG. 11 shows the wavefront aberration before and after four levels of correction. In order to correct a large wavefront aberration as shown before correction, it is necessary to finely set the level of the phase distribution correction amount of the transmitted wavefront. However, if the level is set finely, since the pupil diameter is constant, the width of each level becomes narrow and processing becomes difficult. Further, the corrected wavefront aberration remains a sawtooth wavefront aberration, and the rms value does not become 0.07λ or less.

  Furthermore, such optical elements are mainly made of glass or plastic molds, but especially for light sources with short wavelengths, the refractive index changes greatly due to wavelength variations due to light source wavelength variations and environmental temperature fluctuations, resulting in a phase difference. As a result, a wavefront aberration change (chromatic aberration) occurs. In addition, such a phase conversion element cannot correct wavefront aberrations newly generated due to an axial deviation between the phase conversion element and the optical system.

  Next, the case of the phase conversion liquid crystal element of Patent Document 2 will be described. For example, liquid crystal molecules have a birefringence of 0.2, a liquid crystal layer thickness of 5 μm, and an acquired phase difference at an applied voltage of 4 V is 0.7 μm (= 1λ at 660 nm) (FIG. 12: from application of liquid crystal in an optical disk head). . Considering that the refractive index of the liquid crystal molecules is 0.2 to 0.25 and the power source used for the optical pickup is mainly 5 V, wavefront aberration correction in the liquid crystal molecules is limited to 1λ. That is, correction of wavefront aberration using a liquid crystal element is not suitable for correction with a large amount of aberration.

  The case of spherical aberration correction by divergent light in Patent Document 3 will be described. By setting the light emitting point of the DVD so that it becomes divergent light with respect to the objective lens, the spherical aberration of the DVD can be reduced as shown in FIG. However, since the light emitting point of the DVD is fixed, there are restrictions on the layout of the optical system. Further, there is residual aberration due to the difference in magnification of the optical system, and the rms value does not become 0.07λ or less.

  A first object of the present invention is to make it easy to manufacture and to correct a large wavefront aberration.

  In addition, a second object is to correct any wavefront aberration that occurs in the optical system.

  In addition, a third object is to reduce residual aberration caused by correcting a large wavefront aberration.

  In addition, a fourth object is to reduce the chromatic aberration generated in the wavelength selection phase plate that occurs due to the wavelength variation of the light source and the wavelength variation due to the environmental temperature change.

  In addition, the fifth object is to increase the degree of freedom in the layout of the optical system.

  In addition, a sixth object is to obtain a high response speed by reducing the weight of the drive unit of the objective lens.

Wavefront aberration correcting element of the invention of claim 1, wherein, like the desired wavefront aberration for a particular wavelength lambda ₂ of light is obtained, the thickness of the transmissive portion is provided with a phase step of different plurality of areas A wavelength selective phase plate, a liquid crystal layer capable of obtaining arbitrary wavefront aberration, and a pair of electrode layers and a counter electrode layer sandwiching the liquid crystal layer are laminated.

The invention according to claim 2 is the wavefront aberration correcting element according to claim 1, wherein the phase step d for each region in the wavelength selective phase plate is a light having a specific wavelength λ ₁ .
nd = mλ ₁
(Where n is the refractive index, m is an arbitrary integer)
It is.

  According to a third aspect of the present invention, in the wavefront aberration correcting element according to the first aspect, the liquid crystal layer is provided corresponding to each region in the wavelength selective phase plate, and further transmitted through the wavelength selective phase plate. Thus, the wavefront aberration in the direction opposite to the residual aberration generated is set.

  According to a fourth aspect of the present invention, in the wavefront aberration correcting element according to the first aspect, the liquid crystal layer is provided corresponding to each region in the wavelength selective phase plate, and further transmits through the wavelength selective phase plate. A wavefront aberration in the direction opposite to the chromatic aberration generated in the wavelength selection phase plate due to the wavelength shift of light is set.

  According to a fifth aspect of the present invention, in the wavefront aberration correction element according to any one of the first to fourth aspects, a voltage applied between a pair of electrode layers sandwiching the liquid crystal layer and a counter electrode layer is the wavelength selection element. It differs corresponding to each region in the phase plate.

  According to a sixth aspect of the present invention, in the wavefront aberration correcting element according to any one of the first to fourth aspects, the refractive index of the liquid crystal layer differs corresponding to each region in the wavelength selective phase plate.

  According to a seventh aspect of the present invention, in the wavefront aberration correction element according to any one of the first to fourth aspects, the thickness of the liquid crystal layer is different corresponding to each region in the wavelength selective phase plate.

  An optical pickup according to an eighth aspect of the present invention includes a plurality of light sources that emit light of different wavelengths, an objective lens for condensing a light beam on an information recording medium, and a light beam emitted from each light source in the same optical path. An illumination optical system for combining and entering the same objective lens, a light receiving element for detecting light reflected by the information recording medium, and detection for guiding the light reflected by the information recording medium to the light receiving element An optical system, and a wavefront aberration correction element according to claim 1 provided in an optical path between the objective lens and the illumination optical system.

  According to a ninth aspect of the present invention, in the optical pickup according to the eighth aspect, the illumination optical system or the detection optical system includes a wavelength detection element that detects a wavelength change amount of the light source, and outputs the wavelength detection element. Accordingly, liquid crystal control means for controlling the liquid crystal drive amount of the wavefront aberration correction element is provided.

  According to a tenth aspect of the present invention, in the optical pickup according to the eighth aspect, the wavelength selection phase plate of the wavefront aberration correcting element is only for a light beam having a longer wavelength of two or more light beams having different wavelengths. The phase distribution of the transmitted wavefront is set to change.

  The invention according to claim 11 is the optical pickup according to claim 8, wherein the light source is a short wavelength light beam having a wavelength of 380 to 420 nm, a medium light beam having a wavelength of 640 to 680 nm, and a long wavelength having a wavelength of 760 to 800 nm. The wavelength selective phase plate of the wavefront aberration correction element does not change the phase distribution of the transmitted wavefront with respect to the short-wavelength light beam and the long-wavelength light beam. The phase distribution of the transmitted wavefront is set to change with respect to the medium wavelength light beam, and the illumination optical system for the long wavelength light beam causes the light beam to diverge with respect to the objective lens. It is set to enter.

  According to a twelfth aspect of the present invention, in the optical pickup according to the eleventh aspect, wavefront aberration is corrected by the liquid crystal layer of the wavefront aberration correcting element with respect to the long-wavelength divergent light beam.

  According to a thirteenth aspect of the present invention, in the optical pickup according to the eleventh aspect, the illumination optical system for the medium wavelength light beam is set so that the light beam is incident on the objective lens as substantially parallel light. Yes.

  According to a fourteenth aspect of the present invention, in the optical pickup according to any one of the eighth to thirteenth aspects, the wavefront aberration correcting element is provided on the actuator movable unit that drives the objective lens in a focus direction or a radial direction. And are integrated.

  According to a fifteenth aspect of the present invention, in the optical pickup according to the fourteenth aspect, the wavefront aberration correcting element is provided in an optical path of the illumination optical system different from the actuator movable portion.

  An optical disk apparatus according to a sixteenth aspect is equipped with the optical pickup according to any one of the eighth to fifteenth aspects.

According to the first aspect of the present invention, a large wavefront aberration is reduced by changing the phase distribution of the transmitted wavefront with respect to light of a specific wavelength λ ₂ by the phase step having a plurality of regions where the thickness of the transmission portion is formed. Can be made. Further, any wavefront aberration generated in the optical system can be corrected by the liquid crystal layer stacked on the wavefront aberration correcting element, the pair of electrode layers sandwiching the liquid crystal layer, and the counter electrode layer. In addition, the wavefront aberration correction element shape can be realized with the same size as that of a normal aberration correction element (wavelength selection phase plate / wavefront aberration correction liquid crystal element), so that the optical system does not become large.

According to the second aspect of the present invention, for the light of a certain wavelength λ ₁ different from the specific wavelength λ ₂ , the phase step d for each region in the wavelength selection phase plate is nd = mλ _1. A wavelength-selective wavefront aberration correction element in which a phase difference does not occur with respect to light having the wavelength λ _{1 and} a phase difference occurs with respect to light with the specific wavelength λ ₂ can be obtained.

According to the third aspect of the present invention, residual aberration that occurs when a large wavefront aberration is reduced by the wavelength selective phase plate with respect to the light of the specific wavelength λ ₂ is provided corresponding to each region in the wavelength selective phase plate. By reducing with a liquid crystal layer, it is possible to provide a wavefront aberration correction element with a wide aberration correction range. In addition, since residual aberration can be corrected by the liquid crystal layer provided corresponding to each region in the wavelength selection phase plate, the phase step in the wavelength selection phase plate is reduced, and the manufacture of the wavefront aberration correction element is facilitated. You can also.

According to the fourth aspect of the present invention, the chromatic aberration generated in the wavelength selection phase plate due to the wavelength error is reduced by the liquid crystal layer provided corresponding to each region of the wavelength selection phase plate with respect to the wavelength λ _{1 of the} light source. As a result, the wavelength variation of the light source can be widely allowed. Further, when the wavelength lambda ₁ with the light source varies due to such a change in ambient temperature, by controlling the amount of wavefront aberration created by the liquid crystal layer, thereby improving the reliability of the wavefront aberration correcting element. Similarly, with respect to the specific wavelength λ _{2 of the} light source, the chromatic aberration generated in the wavelength selection phase plate due to the wavelength error is reduced by the liquid crystal layer provided corresponding to each region of the wavelength selection phase plate. Wide variation in wavelength can be allowed. Furthermore, even when the specific wavelength λ _{2 of the} light source fluctuates due to a change in the outside air temperature, the reliability of the wavefront aberration correcting element can be improved by controlling the wavefront aberration generated in the liquid crystal layer.

  According to the fifth aspect of the present invention, the voltage applied between the pair of electrode layers provided opposite to the liquid crystal layer and the counter electrode layer is set to the residual aberration amount of the light transmitted through each region of the wavelength selection phase plate. By changing it accordingly, wavefront aberration in the opposite direction to the residual aberration can be generated, so that the residual aberration can be further reduced.

  According to the invention of claim 6, by changing the refractive index of the liquid crystal layer corresponding to each region in the wavelength selection phase plate, it is possible to generate a wavefront aberration in the opposite direction to the residual aberration. Residual aberrations can be kept small, and the voltage applied between the pair of electrode layers and the counter electrode layer provided with the liquid crystal layer interposed therebetween can be made common in each region. Wiring can be simplified.

  According to the seventh aspect of the invention, by changing the thickness of the liquid crystal layer for each region, the phase amount of the wavefront aberration can be changed according to the amount of residual aberration transmitted through each region. Since the voltage value to be applied can be shared in each region, voltage wiring to the element can be simplified, and the liquid crystal material can be shared in each region, so the materials are unified. In addition, the cost of the element can be reduced.

  According to the invention described in claim 8, by applying the wavefront aberration correction element to an optical pickup having two or more light sources having different wavelengths, the wavefront aberration of each wavelength can be suppressed sufficiently small. The spot diameter of the wavelength light source can be reduced to the diffraction limit even if the substrate thickness is different, and the light flux of the wavelength incident on the objective lens can be corrected even with parallel light incidence, making the optical system layout easy. You can also

  According to the ninth aspect of the present invention, the illumination optical system or the detection optical system includes the wavelength detection element for detecting the wavelength change amount of the light source, and further controls the liquid crystal drive amount according to the output of the wavelength detection element. Therefore, the chromatic aberration that occurs on the phase step surface can always be kept sufficiently small without being affected by the environmental temperature, so the spot diameter of the light source for each wavelength can reach the diffraction limit even if the substrate thickness is different. In addition, since the light flux having a wavelength incident on the objective lens can be corrected even with parallel light incidence, the layout of the optical system can be facilitated.

  According to the tenth aspect of the present invention, the wavefront aberration correction element corrects wavefront aberration generated in an information recording medium having a predetermined substrate thickness without changing the phase distribution of the transmitted wavefront with respect to a light beam having a short wavelength. Thus, since the objective lens can be optimally designed, the best spot diameter can be obtained for a light beam having a short wavelength that easily deteriorates wavefront aberration in the entire optical pickup.

  According to the invention described in claim 11, the same effect as that of the invention described in claim 10 can be obtained for a short-wavelength light beam having a wavelength of 380 to 420 nm and a medium-wavelength light beam having a wavelength of 640 to 680 nm. Further, for a long-wavelength light beam having a wavelength of 760 to 800 nm, the wavefront aberration correction element of the present invention hardly affects the phase distribution of the transmitted wavefront, and is incident on the objective lens by the diverging optical system, thereby making the substrate thickness Since the wavefront aberration of the error can be corrected, the three-wavelength optical system can be reduced in size and a good spot diameter can be obtained.

  According to the twelfth aspect of the present invention, the best spot diameter can be obtained by correcting the wavefront aberration for the long-wavelength divergent light beam by the liquid crystal layer provided in the wavefront aberration correction element.

  According to the thirteenth aspect of the present invention, an optical pickup that is resistant to fluctuations in the optical axis deviation between the objective lens and the incident light beam is realized by making the medium wavelength light beam having a wavelength of 640 to 680 nm incident on the objective lens as substantially parallel light. can do.

  According to the fourteenth aspect of the present invention, since the wavefront aberration correcting element is integrally driven with the objective lens, the optical axis shift between the objective lens and the wavefront aberration correcting element does not occur, so that the best spot diameter can be obtained. .

  According to the fifteenth aspect of the invention, by installing the wavefront aberration correcting element in an optical path different from that of the actuator driving unit, the actuator can be reduced in weight and a high response speed can be obtained. Further, the best spot diameter can be obtained by correcting the deterioration of the wavefront aberration caused by the optical axis shift between the objective lens and the wavefront aberration correcting element by the laminated liquid crystal layer.

  According to the invention described in claim 16, by applying the optical pickup according to any one of claims 9 to 15 to an optical disc apparatus, an optical disc apparatus having high recording / reproducing / erasing quality is obtained because the spot is the best. realizable.

  The best mode for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram showing a basic configuration of an optical pickup 1 in an optical disc apparatus for carrying out the present invention. First, a light source 2 that emits a light beam with a relatively short wavelength and a light source 3 that emits a light beam with a relatively long wavelength are provided, and wavelength synthesis for combining the light beams emitted from these light sources 2 and 3 into the same optical path. A prism 4 is provided, and a common objective lens 6 for condensing and irradiating the light beam on the information recording medium 5 is provided on the optical path of the light beam synthesized by the wavelength synthesis prism 4. The light beam condensed by the objective lens 6 forms a spot on the surface of the information recording medium 5a having the substrate thickness A for the light beam from the light source 2, and the information on the substrate thickness B for the light beam from the light source 3. The spot is set to be formed on the surface of the recording medium 5b.

  A wavefront aberration correction element 7 is disposed on the optical path between the wavelength synthesis prism 4 and the objective lens 6. The objective lens 6 described above is designed so that the wavefront aberration is minimized with respect to the wavelength and substrate thickness A of the light source 2 among the light sources 2 and 3, and the wavelength and substrate thickness B of the other light source 3. The wavefront aberration correction element 7 is effective in the case where a large wavefront aberration is generated for the combination of the wavelength of the light source 3 and the substrate thickness B. It is configured as an aberration correction element.

  A configuration example of the wavefront aberration correcting element 7 will be described with reference to a cross-sectional view shown in FIG. The wavefront aberration correcting element 7 has a laminated structure of a glass layer 8, an electrode layer 9, a liquid crystal layer 10, a counter electrode layer 11, and a wavelength selection phase plate 12.

The thickness of the wavelength selection phase plate 12 has a plurality of steps (levels) having a phase step depending on the region, and each level difference (phase step) d is the wavelength λ _{1 of the} light source 2. It is an integer multiple of. That is,
nd = mλ ₁ (1)
(Where n is the refractive index, m is an arbitrary constant)
It can be expressed by the following formula. For this reason, there is no phase difference with respect to the luminous flux of the light source 2.

  By arbitrarily taking the aberration correction pattern of the electrode layer 9, it is possible to correct the wavefront aberration generated in the optical pickup 1. Examples of the wavefront aberration generated in the optical pickup 1 include coma due to the tilt of the optical disk 5 and astigmatism due to the tilt of the objective lens 6. When correcting an asymmetrical aberration with respect to the center point of the light beam, the pattern of the electrode layer 9 is set according to the shape of the aberration to be corrected. For example, when correcting the coma caused by the inclination of the optical disk 5, the pattern of the electrode layer 9 has the shape shown in FIG. By controlling the applied voltages of S and T, the refractive index distribution of the liquid crystal part is changed and a phase difference is generated, so that coma can be corrected. The aberration correction pattern by such a liquid crystal element can be shared and applied to the light source 1 and the light source 2.

  On the other hand, when the light beam of the light source 3 is incident on the wavelength selection phase plate 12, a stepwise phase difference is received for each region, and the transmitted wavefront becomes a sawtooth-like discontinuous wavefront aberration as shown in FIG. . Such wavefront aberration is referred to as residual aberration due to the wavelength selection phase plate 12. Hereinafter, a correction method for residual aberration will be described.

  In the liquid crystal layer 10, each cell 10 a is configured according to the width of each region where the level of the wavelength selection phase plate 12 changes. That is, the cells 10a are provided corresponding to the respective regions of the wavelength selection phase plate 12, and the width of each level of the wavelength selection phase plate 12 and the width of each cell 10a of the liquid crystal layer 10 are equal. Such a cell configuration may be set as a pattern of the counter electrode layer 11. An example of the structure of the counter electrode pattern 11 in this case is shown in FIG. Also in this case, the width of each cell 11 a is set equal to the width of each level of the wavelength selection phase plate 12.

  With such a configuration, when the light beam of the light source 3 is incident, the wavefront aberration in the direction opposite to the residual aberration generated in the wavelength selection phase plate 12 can be set for each region (each cell 10a or each cell 11a). It becomes. The electrode layer 9 and the counter electrode layer 11 constitute a pair of electrode layers with the liquid crystal layer 10 interposed therebetween, and drive control of the liquid crystal layer 10 based on voltage application by the liquid crystal unit driving device 13.

  FIG. 4 shows the residual aberration generated in the wavelength selection phase plate 12 and the molecular orientation of the liquid crystal molecules caused by each cell 10a or 11a of the liquid crystal layer 10. The refractive index distribution of the liquid crystal layer 10 is changed by changing the molecular orientation of the liquid crystal molecules of the liquid crystal layer 10 so as to cancel the residual aberration generated in the wavelength selection phase plate 12. Here, the molecular orientation is shown as a refractive index ellipsoid whose refractive index is represented by nx, ny. When transmitted through the liquid crystal layer 10, the phase of the light beam, that is, the wavefront is changed, and the residual aberration generated in the wavelength selection phase plate 12 can be canceled.

  FIG. 17 shows the effect of correcting the residual aberration in the wavefront aberration correcting element 7 according to the present embodiment. By generating a wavefront aberration opposite to the residual aberration by the liquid crystal layer 10 and canceling it, the wavefront aberration of the light beam transmitted through the wavefront aberration correcting element 7 can be reduced to 0.02λrms or less.

  When the width (region width) of each level of the wavelength selection phase plate 12 is increased, the residual aberration in the wavelength selection phase plate 12 increases, but the wavefront aberration can be reduced by canceling it with the liquid crystal layer 10. In this case, since the width of each level (region width) of the wavelength selection phase plate 12 and the width of the cell 10a of the liquid crystal layer 10 can be increased, the wavefront aberration correcting element 7 can be easily manufactured.

  Any wavefront aberration and residual aberration generated in the optical pickup 1 can be corrected simultaneously by the liquid crystal layer 10. For this purpose, an electrode pattern matching the wavefront aberration generated in the optical pickup 1 is provided on the electrode layer 9 (or the counter electrode layer 11), and an electrode pattern for correcting residual aberration is provided on the counter electrode layer 11 (or electrode layer). 9). When driving with respect to the light flux of the light source 3, the liquid crystal is driven so as to correct the wavefront aberration in the electrode layer 9 with the residual aberration offset in the counter electrode layer 11. As a result, any wavefront aberration and residual aberration generated in the optical pickup 1 can be corrected simultaneously.

  Incidentally, the residual aberration of the wavelength selection phase plate 12 varies depending on the region of the liquid crystal layer 10. For example, in the residual aberration shown in FIG. 3, the slope (λ / mm) of the residual aberration is four times different between the region a and the region b. When correcting such a wavefront with high accuracy, as shown in FIG. 5, the gradient (V / mm) of the voltage applied by the liquid crystal unit driving device 13 is made different for each region of the liquid crystal layer 10 (each cell 10a). Can be set. In this way, by changing the voltage value applied between the electrode layers 9 and 11 for the liquid crystal layer 10 for each region, the phase change amount of the wavefront aberration can be changed according to the residual aberration amount transmitted through each region (each cell 10a). Since it can be changed, the residual aberration can be further reduced.

  As described above, the wavelength selection phase plate 12 is made of glass or plastic. For this reason, since the depth d is fixed, if the wavelength of the light source deviates from the design wavelength, the refractive index changes and chromatic aberration occurs. Generally, a semiconductor laser is used as a light source, but the wavelength of the semiconductor laser varies depending on a manufacturing section (lot) or a change in environmental temperature. Such chromatic aberration can be corrected by changing the voltage applied to the liquid crystal layer 10. Since the chromatic aberration generated in the wavelength selection phase plate 12 is a wavefront along the shape of the wavelength selection phase plate 12, the pattern of the electrode layer 9 is set according to the width of each region where the level of the wavelength selection phase plate 12 changes. That's fine. Such an electrode pattern is the same as the above-described residual aberration correction pattern, but the method of applying a voltage is different. That is, in the case of the above-described residual aberration correction, a gradient voltage is applied in the cell, whereas in the case of chromatic aberration correction, a uniform voltage is applied in the cell. By combining these, it is possible to correct residual aberrations and simultaneously correct chromatic aberrations generated in the wavelength selection phase plate 12. The configuration in that case is shown in FIG. The applied voltage per cell in correcting the residual aberration may be a difference between the potentials x and y. What is necessary is just to give an offset to the applied voltage of x and y for the applied voltage per cell when correcting chromatic aberration. By providing these simultaneously, it is possible to correct chromatic aberration while correcting residual aberration generated in the wavelength selection phase plate 12.

  When the voltage value for each region of the liquid crystal layer 10 (for each cell 10a) cannot be changed, the same effect can be obtained even if the refractive index of the liquid crystal layer 10 is made different for each region (for each cell 10a). In this case, since the voltage value given to the electrode layers 9 and 11 can be made constant, the electric wire applied to each region (cell 10a) can be shared.

  Further, the same effect can be obtained by changing the thickness of the liquid crystal layer 10 for each region (each cell 10a). In this case, not only can the voltage lines applied to each region (cell 10a) be shared, but also the liquid crystal material can be shared by each region (cell 10a).

  Here, based on such a wavefront aberration correction element 7, FIG. 6 shows a more practical configuration example of the optical pickup 1 than in FIG. As the light source, two or more light sources 2 and 3 having different wavelengths are used. The light beams emitted from the light sources 2 and 3 are combined on the same optical path by the wavelength combining prism 4 constituting the illumination optical system, pass through the wavefront aberration correction element 7 and enter the objective lens 6. The light beam condensed by the objective lens 6 is on the surface of the information recording medium 5a having the substrate thickness A for the light beam having the wavelength of the light source 2, and the information recording medium having the substrate thickness B for the light beam having the wavelength of the light source 2. Spots are formed on the surface 5b. The light reflected by the information recording medium 5 (5a or 5b) carries a signal on the information recording surface and receives light receiving elements 17, 18 by means of detection separating means (beam splitter) 14 and detection lenses 15, 16 constituting a detection optical system. Led to. The detection separation means 14 may be placed independently of the light source (an arrangement example on the light source 2 side in the figure) or may be shared with the light source (an arrangement example on the light source 3 side in the figure).

  The objective lens 6 is designed to minimize the wavefront aberration with respect to the information recording medium 5a having the wavelength of the light source 2 and the substrate thickness A. Since the wavefront aberration correction element 7 of the present embodiment does not change the phase distribution of the transmitted wavefront with respect to the light flux from the light source 2, the wavefront aberration from the light source 2 to the information recording medium 5a having the substrate thickness A is sufficiently small. For this reason, the spot can be narrowed down to the diffraction limit. On the other hand, for the light source 3, the wavefront aberration generated by the combination of the information recording medium 5b having the wavelength of the light source 3 and the substrate thickness B is reduced by the wavefront aberration correcting element 7 of the present embodiment, thereby diffracting the spot. It is possible to squeeze close to the limit.

  In this case, when the wavelength of the light source is shortened, the wavefront aberration of each lens or prism (wavelength synthesis prism 4 or beam splitter 14) is deteriorated. Therefore, the objective lens 6 can be optimally designed for an optical system having a short wavelength, thereby reducing the wavefront aberration of each wavelength and obtaining a favorable spot diameter for each optical system.

  Further, the fluctuation of the wavelength of the light source accompanying the environmental temperature change can be confirmed by providing a temperature sensor or a wavelength detection element (not shown) in or around the optical pickup, and feeding it back to the liquid crystal element driving device. Thus, it is possible to correct chromatic aberration generated in the wavelength selection phase plate 12.

  Next, FIG. 7 shows a configuration of an optical pickup 21 in which the present invention is applied to a three-wavelength light source. First, as a light source, a light source 22 that emits a short wavelength light beam with a wavelength of 380 to 420 nm, a light source 23 that emits a medium wavelength light beam with a wavelength of 640 to 680 nm, and a light source that emits a long wavelength light beam with a wavelength of 760 to 800 nm. 24 is provided. Here, as an example, each of the light sources 23 and 24 is configured as a hologram unit in which corresponding light receiving elements are respectively integrated with a diffraction element that separates outgoing light / returned light by diffraction, and in front of it is a collimator that also serves as a detection lens. Lenses 25 and 26 are provided. On the other hand, the light source 22 side is configured separately from the light receiving element 27, and a collimator lens 28 and a detection lens 29 are disposed in front of each. On the outgoing light path of the light source 22, a detection separation means (beam splitter) 30 that separates the outgoing light and the return light to constitute a detection optical system is disposed. Further, the light flux from the light source 22 and the light source 23 A wavelength synthesizing prism 31 for synthesizing the luminous flux on the same optical path is provided, and a wavelength synthesizing prism 32 for synthesizing the luminous flux from the light sources 22 and 23 and the luminous flux from the light source 24 on the same optical path. An objective lens 6 is disposed on the combined optical path, and a wavefront aberration correction element 7 is disposed on the optical path between the objective lens 6 and the wavelength combining prism 32.

  Here, the objective lens 6 is optimally designed for the short wavelength optical system for the light source 22 having the shortest wavelength. At this time, in the wavefront aberration correcting element 7, the depth of the wavelength selection phase plate 12 is set to 2 × integer multiple of the wavelength λ 1 of the light source 22. By doing so, the depth of the wavelength selection phase plate 12 is an integral multiple of the wavelength λ3 for the light flux of the light source 24 having the wavelength λ3 which is approximately twice the wavelength λ1. Accordingly, the phase distribution of the transmitted wavefront in the wavelength selection phase plate 12 does not change with respect to the light sources 22 and 24. Therefore, when the light sources 22 and 24 emit light, the wavefront aberration correction element 7 of the present embodiment can be obtained by controlling the transmission wavefront phase difference of the liquid crystal layer 10 to be constant within the incident plane. It does not act on the luminous flux. Therefore, the wavefront aberration from the light source 22 to the information recording medium 5a having the substrate thickness A is sufficiently small. For this reason, the spot can be narrowed down to the diffraction limit.

  Further, with respect to the light flux of the light source 23, the wavefront aberration generated by the combination of the information recording medium 5b having the wavelength of the light source 23 and the substrate thickness B is reduced by the wavefront aberration correction element 7 of the present embodiment, thereby reducing the spot. Can be reduced to near the diffraction limit.

  The illumination optical system is set so that the light beam from the light source 24 is divergently incident on the objective lens 6 (for example, the light source 24 is disposed close to the objective lens 6). ). Thereby, the wavefront aberration generated by the combination of the information recording medium 5c having the wavelength of the light source 24 and the substrate thickness C can be reduced by the wavefront aberration generated when the light beam is incident on the objective lens 6 in a divergent manner. it can. Further, the wavefront aberration remaining by such a correction method is corrected by the above-described liquid crystal layer, so that it is possible to narrow the spot to the light source 24 close to the diffraction limit.

  With the above configuration, the optical pickup 21 using the three-wavelength light source can reduce wavefront aberration and narrow the spot to near the diffraction limit in any combination of the light sources 22, 23, and 24-information recording media 5a, 5b, and 5c. It becomes possible.

  Furthermore, in the medium wavelength optical system of the light source 23 having a wavelength of 640 to 680 nm, the optical pickup 21 that is resistant to fluctuations in the optical axis deviation between the objective lens 6 and the incident light beam by making the light beam incident on the objective lens 6 substantially parallel light. Can be realized.

  Further, as shown in FIG. 8, the wavefront aberration correction element 7 of the present embodiment is integrally mounted on the movable portion 34 of the actuator 33 that performs the focusing and tracking operation so as to be integrally driven with the objective lens 6. The optical axis shift between the objective lens 6 and the wavefront aberration correcting element 7 of the present embodiment can be suppressed. For this reason, the wavefront aberration is not deteriorated due to the axis deviation, and the best spot diameter can be obtained.

  In addition, by arranging the wavefront aberration correction element 7 in the optical path of the illumination optical system 4 that is not a movable part of the actuator 33, the weight of the actuator 33 can be reduced. In this case, the deterioration of the wavefront aberration due to the axis deviation due to the tracking operation of the actuator 33 is corrected by the liquid crystal layer 10 provided in the wavefront aberration correction element 7.

  In this way, according to the optical disk apparatus equipped with the optical pickups 1 and 21 of the present invention, since the spots on the information recording medium 5 are optimally narrowed, an optical disk apparatus with high recording / reproducing / erasing quality is provided. It becomes possible to do.

It is a schematic block diagram which shows the basic composition of the optical pick-up in the optical disk device for implementing this invention. It is sectional drawing which shows the structural example of a wavefront aberration correction element. It is a characteristic view which shows the residual aberration by a wavelength selection phase plate. It is a schematic diagram which shows the molecular orientation of the liquid crystal cell which comprises a liquid-crystal layer. It is a characteristic view which shows the applied voltage to the liquid-crystal layer made different for every area | region. It is a block diagram which shows the structural example of a more concrete optical pick-up. It is a block diagram which shows the structural example of the optical pick-up in the case of 3 wavelength structure. It is a side view which shows the example of arrangement | positioning of the wavelength selection phase conversion element of an objective lens integrated drive system. It is a schematic block diagram which shows the structural example of a general optical pick-up. It is a characteristic view which shows the example of a wavefront aberration at the time of using the objective lens for Blu-rays for DVD. It is a characteristic view which shows the wavefront aberration using the wavelength selection phase conversion element of patent document 1. It is a characteristic view which shows the relationship between the applied voltage of a liquid crystal element, and a phase difference. It is a characteristic view which shows the example of correction | amendment of the spherical aberration by the divergent light of patent document 3. It is an electrode pattern in the case of correcting coma aberration in the liquid crystal layer. It is an electrode pattern for correcting residual aberration generated in the wavelength selection phase plate. This is a liquid crystal element structure per cell for correcting residual aberration and chromatic aberration generated in the wavelength selective phase plate. It is a characteristic view which shows the residual aberration correction effect at the time of using the wavefront aberration correction element of one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical pickup 2, 3 Light source 4 Illumination optical system 5 Information recording medium 6 Objective lens 7 Wavefront aberration correction element 9 Electrode layer 10 Liquid crystal layer 11 Counter electrode layer 12 Wavelength selection phase plate 14 Detection optical system 17, 18 Light receiving element 21 Optical pickup 22, 23, 24 Light source 30 Detection optical system 31, 32 Illumination optical system 33 Actuator 34 Actuator movable part

Claims (16)

A wavelength selective phase plate having a phase step composed of a plurality of regions having different thicknesses of the transmission part;
A liquid crystal layer capable of obtaining an arbitrary wavefront aberration, a pair of electrode layers and a counter electrode layer sandwiching the liquid crystal layer, and
A wavefront aberration correction element characterized by being laminated. The phase difference d for each region in the wavelength selective phase plate is for light of a specific wavelength λ ₁ .
nd = mλ ₁
(Where n is the refractive index, m is an arbitrary integer)
The wavefront aberration correction element according to claim 1, wherein:   The liquid crystal layer is provided corresponding to each region in the wavelength selective phase plate, and further sets a wavefront aberration in a direction opposite to the residual aberration generated by transmitting the wavelength selective phase plate. 2. The wavefront aberration correction element according to claim 1, wherein   The liquid crystal layer is provided corresponding to each region in the wavelength selection phase plate, and further has a wavefront in a direction opposite to chromatic aberration generated in the wavelength selection phase plate due to a wavelength shift of light transmitted through the wavelength selection phase plate. The wavefront aberration correction element according to claim 1, wherein aberration is set.   5. The voltage applied between a pair of electrode layers sandwiching the liquid crystal layer and the counter electrode layer is different corresponding to each region in the wavelength selection phase plate. 6. The wavefront aberration correction element described.   5. The wavefront aberration correction element according to claim 1, wherein a refractive index of the liquid crystal layer is different corresponding to each region in the wavelength selection phase plate. 6.   5. The wavefront aberration correction element according to claim 1, wherein a thickness of the liquid crystal layer is different corresponding to each region in the wavelength selection phase plate. 6. A plurality of light sources each emitting light of a different wavelength;
An objective lens for condensing the luminous flux on the information recording medium;
An illumination optical system for combining light beams emitted from the respective light sources into the same optical path and entering the same objective lens,
A light receiving element for detecting light reflected by the information recording medium;
A detection optical system for guiding the light reflected by the information recording medium to the light receiving element;
The wavefront aberration correction element according to any one of claims 1 to 7, provided in an optical path between the objective lens and the illumination optical system;
An optical pickup comprising: In the illumination optical system or detection optical system, comprising a wavelength detection element for detecting the amount of wavelength change of the light source,
9. The optical pickup according to claim 8, further comprising liquid crystal control means for controlling a liquid crystal driving amount of the wavefront aberration correction element in accordance with an output of the wavelength detection element.   The wavelength selection phase plate of the wavefront aberration correction element is set so that the phase distribution of the transmitted wavefront changes only for a light beam having a longer wavelength among two or more light beams having different wavelengths. 9. The optical pickup according to claim 8, wherein The light source includes three light sources that respectively emit a short wavelength light beam with a wavelength of 380 to 420 nm, a medium light beam with a wavelength of 640 to 680 nm, and a long wavelength light beam with a wavelength of 760 to 800 nm.
The wavelength selective phase plate of the wavefront aberration correction element does not change the phase distribution of the transmitted wavefront for the short wavelength light beam and the long wavelength light beam, and the transmitted wavefront phase for the medium wavelength light beam. The distribution is set to change,
9. The optical pickup according to claim 8, wherein the illumination optical system for the long wavelength light beam is set so that the light beam is incident on the objective lens in a divergent manner.   12. The optical pickup according to claim 11, wherein wavefront aberration is further corrected by the liquid crystal layer of the wavefront aberration correcting element for the long wavelength divergent light beam.   12. The optical pickup according to claim 11, wherein the illumination optical system for the medium wavelength light beam is set so that the light beam is incident on the objective lens as substantially parallel light.   14. The wavefront aberration correcting element is provided integrally with the objective lens in an actuator movable portion that drives the objective lens in a focus direction and a radial direction. One optical pickup.   The optical pickup according to claim 9, wherein the wavefront aberration correction element is provided in an optical path of the illumination optical system different from the actuator movable portion. 16. An optical disk device on which the optical pickup according to claim 8 is mounted.

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