JP2004213892A - Method of detecting focal position deviation and optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of detecting a focal position deviation which is capable of exactly matching the focus of a beam-condensing optical system to an information recording layer of an optical recording medium by detecting the focal deviation of the beam-condensing optical system which has no offset. <P>SOLUTION: The focal position deviation of a two-element objective lens 9 is detected in accordance with the light beam passing a region 2b which is a region occupying 60 to 85% of the effective diameter of the beam determined by the aperture of the objective lens around an optical axis OZ of the light beam which has passed the two-elements objective lens 9. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a focus position shift detecting method for detecting a focus position shift occurring in a focusing optical system, and an optical pickup device to which the focus position shift detecting method is applied.

  In recent years, there has been a demand for increasing the recording density of optical discs as the amount of information increases. Higher recording densities of optical disks have been achieved by increasing the linear recording density in the information recording layer of the optical disk and by narrowing the track pitch. In order to cope with the increase in the recording density of the optical disk, it is necessary to reduce the beam diameter of the light beam focused on the information recording layer of the optical disk.

  As a method of reducing the beam diameter of the light beam, it is necessary to increase the numerical aperture (NA) of the light beam emitted from an objective lens serving as a condensing optical system of an optical pickup device for recording and reproducing an optical disk, It is possible to shorten the wavelength of the beam.

  Shortening the wavelength of the light beam is considered to be feasible by changing the light source from a red semiconductor laser to a blue-violet semiconductor laser that has been fully commercialized.

  On the other hand, as a method of realizing a high numerical aperture objective lens, a method of realizing a high numerical aperture by combining an objective lens with a hemispherical lens and forming the objective lens with two lenses (two-group lens) is proposed. Have been.

  Generally, in an optical disc, the information recording layer is covered with a cover glass in order to protect the information recording layer from dust and scratches. Therefore, the light beam transmitted through the objective lens of the optical pickup device passes through the cover glass, and is condensed and focused on the information recording layer thereunder.

When the light beam passes through the cover glass, a spherical aberration (SA: Spherical Aberration) occurs. The spherical aberration SA is
SA ∝ d ・ NA ⁴・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1)
Which is proportional to the thickness d of the cover glass and the fourth power of the NA of the objective lens. Usually, the objective lens is designed to cancel the spherical aberration, so that the spherical aberration of the light beam passing through the objective lens and the cover glass is sufficiently small.

  However, if the thickness of the cover glass deviates from a predetermined value, a spherical aberration occurs in the light beam focused on the information recording layer, the beam diameter increases, and information is read and written correctly. A problem arises that it becomes impossible to do so.

  From the above equation (1), it can be seen that as the thickness error Δd of the cover glass increases, the error ΔSA of the spherical aberration increases and the information cannot be read and written correctly.

  In addition, as a multilayer optical disc formed by laminating information recording layers so that the density of recorded information can be increased in the thickness direction of the optical disc, for example, a DVD (Digital Veratile Disc) having two information recording layers is used. ) Has already been commercialized. In an optical pickup device for recording and reproducing such a multilayer optical disk, it is necessary to focus a light beam on each information recording layer of the optical disk sufficiently small.

  In a multi-layer optical disc having the above-mentioned information recording layer, since the thickness from the surface of the optical disc (cover glass surface) to each information recording layer is different, the spherical aberration generated when the light beam passes through the cover glass of the optical disc. However, it differs for each information recording layer. In this case, for example, the difference (error ΔSA) of the spherical aberration generated in the adjacent information recording layer is proportional to the interlayer distance t (corresponding to d) of the adjacent information recording layer from Expression (1).

  In a DVD having two information recording layers, the NA of the objective lens of the optical pickup device is as small as about 0.6. Therefore, from the above equation (1), even if the cover glass thickness error Δd is slightly increased, the spherical aberration can be reduced. It can be seen that the influence on the error ΔSA is small.

  Therefore, in a conventional DVD device using an optical pickup device having a numerical aperture NA of about 0.6, the error ΔSA of the spherical aberration caused by the thickness error Δd of the cover glass of the DVD is small, and it is difficult to collect for each information recording layer. The light beam to be emitted can be focused sufficiently small.

  However, even if the thickness error Δd of the cover glass is equal, a larger spherical aberration SA occurs as the NA increases. For example, when NA = 0.85 compared to NA = 0.6, approximately four times the spherical aberration SA occurs. Therefore, from the above equation (1), it can be seen that the higher the NA, such as NA = 0.85, the greater the spherical aberration caused by the thickness error of the cover glass.

  Similarly, in the case of a multilayer optical disc, even if the interlayer distance t between adjacent information recording layers is equal, a larger difference (error ΔSA) in spherical aberration occurs as the NA of the objective lens of the optical pickup device increases. For example, when NA = 0.85 compared to NA = 0.6, a difference of about four times the spherical aberration occurs. Therefore, from the above equation (1), it can be seen that the higher the NA, such as NA = 0.85, the greater the difference in spherical aberration for each information recording layer.

  Therefore, in an objective lens having a high NA, the influence of the error of the spherical aberration cannot be ignored, and there is a problem that the accuracy of reading information is reduced. Therefore, it is necessary to correct spherical aberration in order to achieve a high recording density using a high NA objective lens.

  Therefore, as a method of detecting and correcting spherical aberration, for example, Japanese Patent Application Laid-Open No. 2000-171346 discloses an optical pickup device that detects and corrects the above-described spherical aberration. In this optical pickup device, when the light beam is focused on the information recording layer of the optical disk, the focusing position of the light beam differs between the beam near the optical axis and the beam outside the vicinity of the optical axis due to spherical aberration. I use.

  According to the optical pickup device disclosed in the above publication, a light beam to be detected is separated into a light beam near the optical axis of the light beam and a light beam outside the vicinity of the optical axis by an optical element such as a hologram. The shift of the focusing position from the information recording layer in one of the light beams is detected, the spherical aberration is corrected based on the detection result, and the diameter of the light beam focused on each information recording layer of the optical disk is sufficiently increased. Can be smaller.

  In the above-described optical pickup device, for adjusting the focal position of the optical system, for example, half of the light beam is separated by a hologram or the like and focused on a two-segment photodetector. Then, the difference between the two photodetectors is detected as a signal of the focal position shift, and the focal position shift is corrected based on the signal. This method is generally called a beam size method.

  However, in the optical pickup device disclosed in the above publication, when applied to an optical recording / reproducing device using a high numerical aperture lens or a short wavelength light source in order to achieve a high recording density, the above-described optical pickup device is used due to spherical aberration. An offset is generated in the focus position shift signal detected as described above. For this reason, the diameter of the light beam cannot be sufficiently reduced on the information recording layer of the optical recording medium, and it becomes impossible to reproduce information from the optical recording medium or record information on the optical recording medium. Problems arise.

  SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to detect a defocus of a focusing optical system having no offset and to focus the focusing optical system on an information recording layer of an optical recording medium. Is to provide a focus position shift detection method capable of accurately adjusting the focus position and an optical pickup device to which the focus position shift detection method is applied.

  In order to solve the above-mentioned problem, the focus position detecting method of the present invention transmits a light beam reflected from an information recording layer of an optical recording medium and passing through a condensing optical system including an objective lens through an aperture of the objective lens. A first circle or arc having a diameter larger than 70% of the defined effective diameter of the light beam, and a diameter smaller than 70% of the effective diameter of the light beam defined by the aperture of the objective lens; The light beam in a region surrounded by the second circle or the arc and having a larger area than the region of 70% or less of the effective diameter of the light beam is separated from the region of 70% or more of the effective diameter of the light beam. The separated light beam is electrically converted to generate a first focus error signal, and the first focus error signal is used as a focus position shift signal indicating a focus position shift of the focusing optical system.

In order to solve the above-mentioned problem, the focus position detecting method of the present invention provides the first focus error signal as F1 and the focus of a light beam obtained by passing through a region located inside the second circle or arc. The second focus error signal obtained by detecting the position shift is F2, and the third focus obtained by detecting the focus position shift of the light beam obtained by passing through the area located outside the first circle or the arc. Assuming that the error signal is F3, a spherical aberration error signal SAES indicating a spherical aberration generated in the focusing optical system is
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
Or
SAES = F3−F2 × K3 (K3 is a coefficient).

  In order to solve the above problems, an optical pickup device according to the present invention includes a light source, a condensing optical system including an objective lens for condensing a light beam emitted from the light source on an optical recording medium, and the optical recording medium. The first of which has a diameter larger than 70% of the effective diameter of the light beam defined by the aperture of the objective lens centered on the optical axis of the light beam reflected from the information recording layer and passing through the condensing optical system. And a second circle or arc having a diameter smaller than 70% of the effective beam diameter defined by the aperture of the objective lens, and 70% of the effective beam diameter. A light beam separating means for separating a light beam in a region where the above-mentioned region occupies a larger portion than a region having a light beam effective diameter of 70% or less, and electrically converting the light beam separated by the light beam separating device. And the first focus error A first signal generating means for generating a No., the first focus error signal, is characterized in that a focus error signal indicating a focus position deviation of the focusing optical system.

In order to solve the above-mentioned problems, the optical pickup device of the present invention is capable of reducing the focal position shift of a light beam obtained by passing through a region of the light beam separating means located inside the second circle or the circular arc. A second signal generating means for detecting and generating a second focus error signal, and a focal position shift of a light beam obtained by passing through a region of the light beam separating means located outside the first circle or arc. And a third signal generation means for generating a third focus error signal by detecting the first focus error signal F1, the second focus error signal F2, and the third focus error signal F3. At this time, a spherical aberration error signal SAES indicating a spherical aberration generated in the condensing optical system is
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
Or
SAES = F3−F2 × K3 (K3 is a coefficient), and a spherical aberration detecting unit is provided, which is obtained by an equation.

  In order to solve the above-mentioned problems, in the optical pickup device of the present invention, when the optical recording medium has a plurality of information recording layers, the focal position deviation detecting means detects a focal position deviation in each information recording layer. It is characterized by doing.

  According to another aspect of the present invention, there is provided an optical pickup device that corrects a focal position shift of the focusing optical system based on a detection result detected by the focal position shift detecting unit. And spherical aberration correction means for correcting spherical aberration generated in the condensing optical system based on the detection result detected by the spherical aberration detection means, wherein the spherical aberration correction means The correction unit corrects the spherical aberration of the light collecting optical system in a state where the focal position shift of the light collecting optical system is corrected.

  In order to solve the above-mentioned problems, the optical pickup device of the present invention is characterized in that the light beam separating means is a hologram.

  In order to solve the above-mentioned problems, the focus position detection method of the present invention represented by a curve the wavefront of the light beam that has passed through the light-collecting optical system with the light-collecting optical system adjusted to the best image point. At this time, it is characterized in that the focal position shift of the condensing optical system is detected based on an extreme value of the curve and a light beam including a region near the extreme value.

  Here, when a wavefront in a state where the light-collecting optical system is adjusted to the best image point is represented by a curve, a tangent at an extreme value of the curve is substantially parallel to a tangent of a curve representing an ideal wavefront having no spherical aberration. . This means that the light beam passing through the extremum of the curve representing the wavefront when the light beam becomes the best image point on the information recording layer of the optical recording medium converges, and the best image point Indicate that they almost match.

  Therefore, when the wavefront of the light beam that has passed through the focusing optical system and the focusing optical system is adjusted to the best image point is represented by a curve as in the above configuration, the extreme value of the curve and the extreme value If the focal position shift of the focusing optical system is detected based on the light beam corresponding to the area near the value, even if spherical aberration occurs in the focusing optical system, the influence of the spherical aberration is reduced. The focus position deviation can be accurately detected with little reception.

  This makes it possible to optically detect a shift in the focal position of the light-collecting optical system without any offset, so that it is possible to appropriately correct the shift in the focal position of the light-collecting optical system. It is possible to accurately focus the focusing optical system on the recording layer.

  According to another aspect of the present invention, there is provided a method of detecting a focal position, the method comprising defining an aperture of the objective lens around an optical axis of a light beam passing through a condensing optical system including an objective lens. The focus position shift of the condensing optical system is detected based on a light beam in a region of 60 to 85% of the effective diameter of the light beam.

  Here, with respect to the optical axis of the light beam that has passed through the condensing optical system including the objective lens, a light beam in a region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens converges. The convergence point (focal point) substantially coincides with the best image point of the light beam that has passed through the focusing optical system.

  Therefore, as in the above-described configuration, an area of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, centered on the optical axis of the light beam that has passed through the condensing optical system including the objective lens. If the focal position shift of the above-mentioned condensing optical system is detected based on the light beam, even if spherical aberration occurs in the condensing optical system, the focal position is hardly affected by the spherical aberration. The displacement can be accurately detected.

  This makes it possible to optically detect a shift in the focal position of the light-collecting optical system without any offset, so that it is possible to appropriately correct the shift in the focal position of the light-collecting optical system. It is possible to accurately focus the focusing optical system on the recording layer.

  A light beam in a region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens is separated from the light beam passing through the condensing optical system, and the separated light beam is electrically converted. Then, a first focus error signal may be generated, and the first focus error signal may be used as a focus position shift signal indicating a focus position shift of the light collecting optical system.

  In this case, a light beam separated from the light beam that has passed through the condensing optical system is electrically converted to generate a first focus error signal, and this first signal is converted to a focus indicating a focal position shift of the condensing optical system. By using the position shift signal, the focus position shift of the light collecting optical system can be detected by an electric signal.

  As a result, the obtained electric signal can be used as it is in the drive circuit for controlling the drive of the focusing optical system, so that the focus position deviation can be easily corrected by driving the focusing optical system to an appropriate position. can do.

  The separated light beam is focused on the optical axis of the light beam passing through the condensing optical system and has a diameter larger than 85% of the effective diameter of the light beam defined by the aperture of the objective lens. The light beam separating means has a region surrounded by one circle or arc and a second circle or arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens. The region may be obtained by passing a light beam that has passed through the light-collecting optical system.

  In this case, a first circle or arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, with the optical axis of the light beam passing through the condensing optical system as a center. By using a light beam separating means having a region surrounded by a second circle or an arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens, It is possible to easily separate and obtain a light beam required for the shift of the focal position of the optical optical system.

  A second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the arc in the light beam separating means; Alternatively, based on at least one of a third focus error signal and a third focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located outside the circular arc, the spherical surface of the condensing optical system is used. The aberration may be detected.

  In this case, a first circle or a circular arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens around the optical axis of the light beam passed through the condensing optical system; As described above, the light beam passing through the area surrounded by the second circle or the arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens has a spherical aberration as described above. Coincides with the best image point without being affected by

  Accordingly, a second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the circular arc, which is a region other than the above-mentioned region, The third focus error signal obtained by detecting the focal position shift of the light beam obtained by passing through a region outside the one circle or arc is affected by spherical aberration.

  Thus, the spherical aberration of the condensing optical system can be accurately detected by using either one of the second focus error signal and the third focus error signal.

A focus position shift signal indicating the focus position shift of the light condensing optical system is defined as the first focus error signal, the first focus error signal is F1, the second focus error signal is F2, and the third focus error signal is F3. And when
A spherical aberration error signal SAES indicating a spherical aberration generated in the light-collecting optical system is:
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
May be obtained by the formula shown in the following.

  In this case, the first focus error signal F2 and the third use point error signal F3 used for detecting the spherical aberration of the condensing optical system include the first signal for detecting the focal position shift of the condensing optical system. Since the error signal is taken into account, the spherical aberration can be detected while suppressing the influence of the focal position shift.

  In order to solve the above-described problems, an optical pickup device of the present invention has passed through a light source, a condensing optical system for condensing a light beam emitted from the light source on an optical recording medium, and the condensing optical system. When the wavefront of the light beam when the light beam becomes the best image point on the information recording layer of the optical recording medium is represented by a curve, the extreme value of the curve and the light beam corresponding to the region near the extreme value And a focus position shift detecting means for detecting a focus position shift of the condensing optical system based on the above.

  Here, when the wavefront when the light beam becomes the best image point on the information recording layer of the optical recording medium is represented by a curve, the tangent at the extremum of this curve is the tangent to the curve representing the ideal wavefront without spherical aberration. Is almost parallel to This means that the light beam passing through the extremum of the curve representing the wavefront when the light beam becomes the best image point on the information recording layer of the optical recording medium converges, and the best image point Indicate that they almost match.

  Therefore, when the wavefront of the light beam that has passed through the light-collecting optical system and becomes the best image point on the information recording layer of the optical recording medium is represented by a curve as in the above configuration, If the focal position deviation of the focusing optical system is detected based on the extreme value of the light beam and the light beam corresponding to the region near the extreme value, even if spherical aberration occurs in the focusing optical system, It is possible to optically detect the focal position shift of the light collecting optical system without any offset.

  This makes it possible to appropriately correct the focal position shift of the light-collecting optical system without the influence of spherical aberration, so that the light-collecting optical system can accurately focus on the information recording layer of the optical recording medium. As a result, information can be recorded and reproduced on the optical recording medium satisfactorily at all times.

  According to another embodiment of the present invention, there is provided an optical pickup apparatus comprising: a light source; a condensing optical system including an objective lens for condensing a light beam emitted from the light source onto a recording medium; Focusing position of the condensing optical system based on the light beam in the region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens with the optical axis of the light beam passing through the optical system as the center. A focus position shift detecting means for detecting a shift.

  Here, with respect to the optical axis of the light beam that has passed through the condensing optical system including the objective lens, a light beam in a region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens converges. The convergence point (focal point) substantially coincides with the best image point of the light beam that has passed through the focusing optical system.

  Therefore, as in the above-described configuration, an area of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, centered on the optical axis of the light beam that has passed through the condensing optical system including the objective lens. If the focal position shift of the condensing optical system is detected based on the light beam, even if spherical aberration occurs in the condensing optical system, the focal position of the condensing optical system can be optically adjusted without offset. A shift can be detected.

  This makes it possible to appropriately correct the focal position shift of the light-collecting optical system without the influence of spherical aberration, so that the light-collecting optical system can accurately focus on the information recording layer of the optical recording medium. As a result, information can be recorded and reproduced on the optical recording medium satisfactorily at all times.

  The focus position shift detecting means separates a light beam in an area of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens from the light beam passed through the condensing optical system. Means, and first signal generating means for generating a first focus error signal based on the light beam separated by the light beam separating means, and converting the first focus error signal into a focal position of the condensing optical system. A focus error signal indicating a shift may be used.

  In this case, a light beam separated from the light beam that has passed through the condensing optical system is electrically converted to generate a first focus error signal, and this first signal is converted to a focus indicating a focal position shift of the condensing optical system. By using the position shift signal, the focus position shift of the light collecting optical system can be detected by an electric signal.

  As a result, the obtained electric signal can be used as it is in the drive circuit for controlling the drive of the focusing optical system, so that the focus position deviation can be easily corrected by driving the focusing optical system to an appropriate position. can do.

  The light beam separating means has a first center having an optical axis of the light beam passing through the condensing optical system and having a diameter larger than 85% of an effective diameter of the light beam defined by the aperture of the objective lens. And a first area surrounded by a second circle or arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens. The first signal generating means may generate a first focus error signal based on the light beam passing through the first area of the light beam separating means.

  In this case, a first circle or arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, with the optical axis of the light beam passing through the condensing optical system as a center. By using a light beam separating means having a region surrounded by a second circle or an arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens, It is possible to easily separate and obtain a light beam required for the shift of the focal position of the optical optical system.

  A second signal generating means for detecting a focal position shift of a light beam obtained by passing through an area located inside the second circle or the circular arc and generating a second focus error signal; And at least one of a third signal generator for detecting a focal position shift of a light beam obtained by passing through a region located outside the circle or the arc and generating a third focus error signal. And a spherical aberration detecting means for detecting a spherical aberration of the condensing optical system based on at least one of the second focus error signal and the third focus error signal.

  In this case, a first circle or a circular arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens around the optical axis of the light beam passed through the condensing optical system; As described above, the light beam passing through the area surrounded by the second circle or the arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens has a spherical aberration as described above. Coincides with the best image point without being affected by

  Accordingly, a second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the circular arc, which is a region other than the above-mentioned region, The third focus error signal obtained by detecting the focal position shift of the light beam obtained by passing through a region outside the one circle or arc is affected by spherical aberration.

  Thus, the spherical aberration of the condensing optical system can be accurately detected by using either one of the second focus error signal and the third focus error signal.

The spherical aberration detecting means sets a focus error signal generated in the light-collecting optical system as the first focus error signal, the first focus error signal as F1, the second focus error signal as F2, and the third focus error signal. Is F3, a spherical aberration error signal SAES indicating a spherical aberration generated in the condensing optical system is
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
May be obtained by the equation shown in

  In this case, the first focus error signal F2 and the third use point error signal F3 used for detecting the spherical aberration of the condensing optical system include the first signal for detecting the focal position shift of the condensing optical system. Since the error signal is taken into account, the spherical aberration can be detected while suppressing the influence of the focal position shift.

  When the optical recording medium has a plurality of information recording layers, the focal position deviation detecting means may detect a focal position deviation in each information recording layer.

  In this case, even if a spherical aberration occurs in the condensing optical system due to a difference in the distance from the light incident side surface of the optical recording medium to each information recording layer, each information recording layer is not affected by these spherical aberrations. It is possible to accurately detect a focus position shift.

  Further, based on a detection result detected by the focal position deviation detecting means, a focal position deviation correcting means for correcting a focal position deviation of the focusing optical system, and based on a detection result detected by the spherical aberration detecting means. And a spherical aberration correcting means for correcting spherical aberration generated in the condensing optical system, wherein the spherical aberration correcting means corrects a focal position shift of the condensing optical system by the focal position shift correcting means. Alternatively, the spherical aberration of the condensing optical system may be corrected.

  In this case, the spherical aberration of the light collecting optical system is corrected in a state where the focal position shift of the light collecting optical system is corrected by the focal position shift correcting means by the spherical aberration correcting means, so that the information recording layer of the optical recording medium can be corrected. Thus, the spherical aberration can be corrected while maintaining the light beam diameter in the best condition.

  As described above, the focal position detection method of the present invention is such that, when a wavefront of the light beam that has passed through the condensing optical system and the condensing optical system is adjusted to the best image point is represented by a curve, this curve The focal position shift of the above-mentioned condensing optical system is detected on the basis of the extreme value and the light beam corresponding to the region near the extreme value.

  Therefore, when the wavefront of the light beam that has passed through the condensing optical system and the condensing optical system is adjusted to the best image point is represented by a curve, the extremum of the curve and the region near the extremum are represented by a curve. If the focal position shift of the above-mentioned condensing optical system is detected based on the corresponding light beam, even if spherical aberration occurs in the condensing optical system, the focal point of the condensing optical system can be optically adjusted without offset. Positional deviation can be detected.

  Therefore, since the focal position shift of the condensing optical system can be appropriately corrected, there is an effect that the focus of the condensing optical system can be accurately adjusted to the information recording layer of the optical recording medium.

  According to another focus position detecting method of the present invention, as described above, the light beam defined by the aperture of the objective lens is centered on the optical axis of the light beam passing through the focusing optical system including the objective lens. The configuration is such that the focal position shift of the condensing optical system is detected based on the light beam in the region of 60 to 85% of the effective diameter.

  Therefore, based on the light beam in the region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, centered on the optical axis of the light beam passing through the condensing optical system including the objective lens. By detecting the focal position shift of the condensing optical system, it is possible to optically detect the focal position shift of the condensing optical system without offset even if spherical aberration occurs in the condensing optical system. it can.

  Therefore, since the focal position shift of the condensing optical system can be appropriately corrected, there is an effect that the focus of the condensing optical system can be accurately adjusted to the information recording layer of the optical recording medium.

  A light beam in a region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens is separated from the light beam passing through the condensing optical system, and the separated light beam is electrically converted. Then, a first focus error signal may be generated, and the first focus error signal may be used as a focus position shift signal indicating a focus position shift of the light collecting optical system.

  In this case, a light beam separated from the light beam that has passed through the condensing optical system is electrically converted to generate a first focus error signal, and this first signal is converted to a focus indicating a focal position shift of the condensing optical system. By using the position shift signal, the focus position shift of the light collecting optical system can be detected by an electric signal.

  As a result, the obtained electric signal can be used as it is in the drive circuit for controlling the drive of the focusing optical system, so that the focus position deviation can be easily corrected by driving the focusing optical system to an appropriate position. It has the effect that it can be done.

  The separated light beam is focused on the optical axis of the light beam passing through the condensing optical system and has a diameter larger than 85% of the effective diameter of the light beam defined by the aperture of the objective lens. The light beam separating means has a region surrounded by one circle or arc and a second circle or arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens. The region may be obtained by passing a light beam that has passed through the light-collecting optical system.

  In this case, a first circle or arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, with the optical axis of the light beam passing through the condensing optical system as a center. By using a light beam separating means having a region surrounded by a second circle or an arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens, There is an effect that a light beam required for the shift of the focal position of the optical optical system can be easily separated and obtained.

  A second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the arc in the light beam separating means; Alternatively, based on at least one of a third focus error signal and a third focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located outside the circular arc, the spherical surface of the condensing optical system is used. The aberration may be detected.

  In this case, a first circle or a circular arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens around the optical axis of the light beam passed through the condensing optical system; As described above, the light beam passing through the area surrounded by the second circle or the arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens has a spherical aberration as described above. Coincides with the best image point without being affected by

  Accordingly, a second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the circular arc, which is a region other than the above-mentioned region, The third focus error signal obtained by detecting the focal position shift of the light beam obtained by passing through a region outside the one circle or arc is affected by spherical aberration.

  Thus, using either one of the second focus error signal and the third focus error signal described above has an effect that the spherical aberration of the light collecting optical system can be accurately detected.

A focus position shift signal indicating the focus position shift of the light condensing optical system is defined as the first focus error signal, the first focus error signal is F1, the second focus error signal is F2, and the third focus error signal is F3. And when
A spherical aberration error signal SAES indicating a spherical aberration generated in the light-collecting optical system is:
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
May be obtained by the formula shown in the following.

  In this case, the second focus error signal F2 and the third use point error signal F3 used for detecting the spherical aberration of the condensing optical system include a first signal for detecting a focal position shift of the condensing optical system. Since the error signal is taken into consideration, there is an effect that the spherical aberration can be detected while suppressing the influence of the focal position shift.

  As described above, the optical pickup device of the present invention includes a light source, a condensing optical system that condenses a light beam emitted from the light source on an optical recording medium, and a light beam that has passed through the condensing optical system. When the wavefront when the light beam becomes the best image point on the information recording layer of the optical recording medium is represented by a curve, based on a light beam corresponding to an extreme value of the curve and a region near the extreme value, A focus position shift detecting means for detecting a focus position shift of the condensing optical system.

  Therefore, when the wavefront of the light beam passing through the focusing optical system when the light beam becomes the best image point on the information recording layer of the optical recording medium is represented by a curve, the extremum of this curve and this If the focal position shift of the above-mentioned condensing optical system is detected based on the light beam corresponding to the region near the extreme value, even if spherical aberration occurs in the condensing optical system, optically without offset It is possible to detect a shift in the focal position of the light collecting optical system.

  This makes it possible to appropriately correct the focal position shift of the light-collecting optical system without the influence of spherical aberration, so that there is an effect that information can be always recorded and reproduced on the optical recording medium satisfactorily. .

  As described above, another optical pickup device according to the present invention includes a light source, a condensing optical system including an objective lens for condensing a light beam emitted from the light source onto a recording medium, and a light passing through the condensing optical system. A focus position shift of the condensing optical system is detected based on a light beam in a region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens with the optical axis of the light beam as a center. This is a configuration including a focus position shift detecting unit.

  Therefore, based on the light beam in the region of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, centered on the optical axis of the light beam passing through the condensing optical system including the objective lens. By detecting the focal position shift of the condensing optical system, it is possible to optically detect the focal position shift of the condensing optical system without offset even if spherical aberration occurs in the condensing optical system. it can.

  This makes it possible to appropriately correct the focal position shift of the light-collecting optical system without the influence of spherical aberration, so that there is an effect that information can be always recorded and reproduced on the optical recording medium satisfactorily. .

  The focus position shift detecting means separates a light beam in an area of 60 to 85% of the effective diameter of the light beam defined by the aperture of the objective lens from the light beam passed through the condensing optical system. Means, and first signal generating means for generating a first focus error signal based on the light beam separated by the light beam separating means, and converting the first focus error signal into a focal position of the condensing optical system. A focus error signal indicating a shift may be used.

  In this case, a light beam separated from the light beam that has passed through the condensing optical system is electrically converted to generate a first focus error signal, and this first signal is converted to a focus indicating a focal position shift of the condensing optical system. By using the position shift signal, the focus position shift of the light collecting optical system can be detected by an electric signal.

  As a result, the obtained electric signal can be used as it is in the drive circuit for controlling the drive of the focusing optical system, so that the focus position deviation can be easily corrected by driving the focusing optical system to an appropriate position. It has the effect that it can be done.

  The light beam separating means has a first center having an optical axis of the light beam passing through the condensing optical system and having a diameter larger than 85% of an effective diameter of the light beam defined by the aperture of the objective lens. And a first area surrounded by a second circle or arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens. The first signal generating means may generate a first focus error signal based on the light beam passing through the first area of the light beam separating means.

  In this case, a first circle or arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens, with the optical axis of the light beam passing through the condensing optical system as a center. By using a light beam separating means having a region surrounded by a second circle or an arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens, There is an effect that a light beam required for the shift of the focal position of the optical optical system can be easily separated and obtained.

  A second signal generating means for detecting a focal position shift of a light beam obtained by passing through an area located inside the second circle or the circular arc and generating a second focus error signal; And at least one of a third signal generator for detecting a focal position shift of a light beam obtained by passing through a region located outside the circle or the arc and generating a third focus error signal. And a spherical aberration detecting means for detecting a spherical aberration of the condensing optical system based on at least one of the second focus error signal and the third focus error signal.

  In this case, a first circle or a circular arc having a diameter larger than a diameter equivalent to 85% of the effective diameter of the light beam defined by the aperture of the objective lens around the optical axis of the light beam passed through the condensing optical system; As described above, the light beam passing through the area surrounded by the second circle or the arc having a diameter smaller than 60% of the effective diameter of the light beam defined by the aperture of the objective lens has a spherical aberration as described above. Coincides with the best image point without being affected by

  Accordingly, a second focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located inside the second circle or the circular arc, which is a region other than the above-mentioned region, The third focus error signal obtained by detecting the focal position shift of the light beam obtained by passing through a region outside the one circle or arc is affected by spherical aberration.

  Thus, using either one of the second focus error signal and the third focus error signal described above has an effect that the spherical aberration of the light collecting optical system can be accurately detected.

The spherical aberration detecting means sets a focus error signal generated in the light-collecting optical system as the first focus error signal, the first focus error signal as F1, the second focus error signal as F2, and the third focus error signal. Is F3, a spherical aberration error signal SAES indicating a spherical aberration generated in the condensing optical system is
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
May be obtained by the equation shown in

  In this case, the first focus error signal F2 and the third use point error signal F3 used for detecting the spherical aberration of the condensing optical system include the first signal for detecting the focal position shift of the condensing optical system. Since the error signal is taken into consideration, there is an effect that the spherical aberration can be detected while suppressing the influence of the focal position shift.

  When the optical recording medium has a plurality of information recording layers, the focal position deviation detecting means may detect a focal position deviation in each information recording layer.

  In this case, even if a spherical aberration occurs in the condensing optical system due to a difference in the distance from the light incident side surface of the optical recording medium to each information recording layer, each information recording layer is not affected by these spherical aberrations. There is an effect that the focal position shift can be accurately detected.

  Further, based on a detection result detected by the focal position deviation detecting means, a focal position deviation correcting means for correcting a focal position deviation of the focusing optical system, and based on a detection result detected by the spherical aberration detecting means. And a spherical aberration correcting means for correcting spherical aberration generated in the condensing optical system, wherein the spherical aberration correcting means corrects a focal position shift of the condensing optical system by the focal position shift correcting means. Alternatively, the spherical aberration of the condensing optical system may be corrected.

  In this case, the spherical aberration of the condensing optical system is corrected in a state where the focal position shift of the condensing optical system is corrected by the focal position shift correcting unit by the spherical aberration correcting unit. Thus, it is possible to correct spherical aberration while maintaining the light beam diameter in the best condition.

  The following will describe one embodiment of the present invention. In the present embodiment, the focus position deviation detecting method of the present invention is applied to an optical pickup device provided in an optical recording / reproducing apparatus for optically recording / reproducing information on / from an optical disk as an optical recording medium. The following describes the example.

  As shown in FIG. 2, an optical recording / reproducing apparatus according to the present embodiment includes a spindle motor 62 for rotatingly driving an optical disk 6 as an optical recording medium, an optical pickup device 10 for recording / reproducing information on / from the optical disk 6, and the spindle motor 62 and a drive control unit 51 for controlling the drive of the optical pickup device 10.

  The optical pickup device 10 has a semiconductor laser 1 as a light source for irradiating an optical disk 6 with a light beam, a hologram 2, a collimator lens 3, a two-element objective lens 9 as a light-collecting optical system, and detection devices 7, 8. are doing.

  A mirror 63 is provided between the two-element objective lens 9 and the collimator lens 3 to refract the optical path of the light beam from the two-element objective lens 9 or the light beam from the collimator lens 3 by about 90 °. .

  Further, the two-element objective lens 9 has a structure in which the first lens element 4 and the second lens element 5 are arranged in this order from the side irradiated with the light beam from the semiconductor laser 1.

  The first lens element 4 is held by a holder 52 at the periphery. A focus actuator 53 and a tracking actuator 64 are provided on the outer periphery of the holder 52.

  The focus actuator 53 moves the two-element objective lens 9 to an appropriate position in the optical axis direction to perform focusing control. The tracking control is performed by moving the two-element objective lens 9 in the radial direction (the direction orthogonal to the direction of the track formed on the optical disc 6 and the direction of the optical axis) by the tracking actuator 64.

  By accurately controlling the driving of the tracking actuator 64, the light beam can be accurately tracked on the information track of the optical disk 6.

  The lens second element 5 is held by a holder 54 at the peripheral edge. A second element actuator 55 for moving the second lens element 5 in the optical axis direction is provided on the inner peripheral surface of the holder 52 facing the outer peripheral portion of the holder 54. By controlling the driving of the second element actuator 55, the distance between the first lens element 4 and the second lens element 5 is adjusted, and spherical aberration generated in the optical system of the optical pickup device 10 is corrected. .

  The drive control unit 51 includes a spindle motor drive circuit 56 for controlling the drive of the spindle motor 62, a focus drive circuit 57 for controlling the drive of the focus actuator 53, and a tracking drive circuit for controlling the drive of the tracking actuator 64. 61, a second element driving circuit 58 for controlling the driving of the second element actuator 55, and a control for generating a control signal to each control circuit from the signals obtained from the detection devices 7 and 8 The signal generating circuit 59 includes an information reproducing circuit 60 for reproducing information recorded on the optical disk 6 from signals obtained from the detection devices 7 and 8 and generating a reproduced signal.

  The control signal generation circuit 59 generates a tracking error signal, a focus error signal FES, and a spherical aberration error signal SAES based on the signals obtained from the detection devices 7 and 8, and the tracking error signal is sent to the tracking drive circuit 61. The focus error signal FES is output to the focus drive circuit 57, and the spherical aberration error signal SAES is output to the second element drive circuit 58. Then, each drive circuit controls the drive of each member based on each error signal.

  For example, when the focus error signal FES is input to the focus drive circuit 57, the two-element objective lens 9 is moved in the optical axis direction based on the value of the FES, and the focus position shift of the two-element objective lens 9 is performed. The drive of the focus actuator 53 is controlled so as to make a correction.

  Further, when the spherical aberration error signal SAES is input, the second element driving circuit 58 moves the second lens element 5 in the optical axis direction based on the value of the SAES, and the optical system of the optical pickup device 10. The driving of the second element actuator 55 is controlled so as to correct the spherical aberration generated in the above. However, when spherical aberration is corrected by the spherical aberration correcting mechanism, the distance between the first lens element 4 and the second lens element 5 of the two-element objective lens 9 is fixed, and the spherical surface input to the spherical aberration correcting mechanism is fixed. The spherical aberration is corrected according to the value of the aberration error signal SAES.

  Here, the details of the optical pickup device 10 will be described below with reference to FIG. For convenience of explanation, in the optical pickup device 10 shown in FIG. 1, the mirror 63 shown in FIG. 2 is omitted.

  In the optical pickup device 10, the hologram 2, the collimating lens 3, the first lens element 4 and the second lens element 5 constituting the two-element objective lens 9 are the light beam irradiation surface of the semiconductor laser 1 and the light beam reflection surface of the optical disk. The detection devices 7 and 8 are arranged at the focal position of the diffracted light of the hologram 2.

  That is, in the optical pickup device 10 having the above-described configuration, the light beam emitted from the semiconductor laser 1 passes through the hologram 2 as 0th-order diffracted light, and is converted into parallel light by the collimating lens 3. The light passes through a two-element objective lens 9 composed of the element 4 and the second lens element 5 and is focused on the information recording layer 6c or 6d on the optical disk 6.

  On the other hand, the light beam reflected from the information recording layer 6c or 6d of the optical disc 6 passes through the respective members of the two-element objective lens 9, in the order of the lens second element 5, the lens first element 4, and the collimating lens 3, and passes through the hologram 2 And is diffracted by the hologram 2 and condensed on the detection devices 7 and 8.

  The detecting device 7 includes a first light receiving portion 7a, a second light receiving portion 7b, and a third light receiving portion 7c. The detecting device 8 includes a fourth light receiving portion 8a and a fifth light receiving portion 8b. The light beam is converted into an electric signal by these detecting devices 7 and 8.

  The optical disk 6 includes a cover glass 6a, a substrate 6b, and two information recording layers 6c and 6d formed between the cover glass 6a and the substrate 6b. That is, the optical disc 6 is a two-layer disc, and the optical pickup device 10 reproduces information from each information recording layer by condensing a light beam on the information recording layer 6c or 6d, and transmits the information to each information recording layer. Is recorded.

  Therefore, in the following description, the information recording layer of the optical disc 6 represents either the information recording layer 6c or 6d, and the optical pickup device 10 focuses a light beam on either information recording layer and records or records information. It can be reproduced.

  The hologram 2 has five regions 2a, 2b, 2c, 2d, and 2e.

  The first area 2a is an area surrounded by a first straight line CL1 orthogonal to the optical axis OZ, a first circle E1 centered on the optical axis OZ, and a second arc E2.

  The second area 2b is an area surrounded by the first straight line CL1, the second arc E2, and the third arc E3 centered on the optical axis OZ.

  The third region 2c is a region surrounded by the first straight line CL1 and the third arc E3.

  The fourth region 2d is a region surrounded by the first straight line CL1, the first circle E1, the optical axis OZ, and the second straight line CL2 orthogonal to the first straight line CL1.

  The fifth region 2e is a region surrounded by the first straight line CL1, the second straight line CL2, and the first circle E1, similarly to the fourth region 2d.

  The hologram 2 transmits the light emitted from the semiconductor laser 1 to the optical disk 6 as the 0th-order diffracted light, diffracts the reflected light from the optical disk 6 and guides the light to the detection devices 7 and 8.

  The hologram 2 is formed so as to diffract the light beam passing through the hologram 2 from the optical disk 6 side and to condense it at different points in each region. That is, of the light beams reflected by the recorded information image on the optical disk 6, the first light beam diffracted in the first area 2a of the hologram 2 forms a condensed spot in the first light receiving portion 7a, and the hologram 2 The second light beam diffracted in the second region 2b forms a converging spot in the second light receiving portion 7b, and the third light beam diffracted in the third region 2c of the hologram 2 is the third light beam. A converging spot is formed by the portion 7c, and the fourth light beam diffracted by the fourth region 2d of the hologram 2 forms a converging spot by the fourth light receiving portion 8a, and is formed by the fifth region 2e of the hologram 2 The diffracted fifth light beam forms a focused spot at the fifth light receiving portion 8b.

  Here, details of the detection devices 7 and 8 will be described below with reference to FIG.

  As shown in FIG. 3, the detecting device 7 is formed by juxtaposing the above three light receiving portions (first light receiving portion 7a, second light receiving portion 7b, and third light receiving portion 7c). The two light receiving sections (the fourth light receiving section 8a and the fifth light receiving section 8b) are formed side by side.

  The first light receiving unit 7a, the second light receiving unit 7b, and the third light receiving unit 7c are respectively provided with two divided photodetectors 11a, 11b and 12a, and 12b and 13a and 13b. Each of the light receiving units is arranged such that converged spots of the first, second, and third light beams are formed on the division line of each photodetector, and converts the light beams into electric signals.

  Each of the fourth light receiving unit 8a and the fifth light receiving unit 8b has one photodetector 14 and 15 respectively, and converts the fourth and fifth light beams into electric signals.

  The electric signal obtained by each of the photodetectors described above is used by the drive control unit 51 (FIG. 2) to shift the focal position of the two-element objective lens 9 and to reproduce information from the optical disc 6. For example, the electric signal is output to the information reproduction circuit 60 (FIG. 2) and is converted into a reproduction signal RF. At this time, the reproduction signal RF recorded on the optical disk 6 is given by the sum of the electric signals output from each photodetector.

  In the optical recording / reproducing apparatus having the above configuration, tracking drive control is performed in order to focus the light beam emitted from the two-element objective lens 9 on a track formed on the optical disk. That is, the tracking actuator 64 (FIG. 2) is driven to move the two-element objective lens 9 in the radial direction (radial direction) of the optical disk 6 so that the light beam is focused on the track.

Here, the tracking shift signal TES indicating the amount by which the focused beam is shifted in the radial direction from the track is obtained by using the electric signals 14S and 15S output from the photodetectors 14 and 15 as TES = 14S-15S.・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (2)
Is represented by

  The method of measuring the tracking error by obtaining the tracking error signal TES by the above equation (2) utilizes the phenomenon that the reflected diffracted light pattern is unbalanced in the radial direction due to the positional relationship between the track and the condensed spot. This is called a so-called push-pull method. Therefore, in order to measure the amount of imbalance, it is desirable that the second straight line CL2 that divides the fourth region 2d and the fifth region 2e of the hologram 2 be orthogonal to the radial direction.

  The defocus correction of the two-element objective lens 9 is performed as follows using the electric signals from the respective photodetectors.

If the focal point does not coincide with the information recording layer, the light beam in one of the first light receiving portion 7a, the second light receiving portion 7b, and the third light receiving portion 7c of the detection device 7 depends on one of the photodetectors. Then, the electric signals from the photodetectors 11a and 11b that convert the diffracted light from the first area 2a of the hologram 2 into electric signals are set as 11aS and 11bS, and the first focus error signal F1 is
F1 = 11aS-11bS (3)
And the electric signals from the photodetectors 12a and 12b that convert the diffracted light from the second area 2b of the hologram 2 into electric signals are 12aS and 12bS, and the second focus error signal F2 is
F2 = 12aS-12bS (4)
And the electric signals from the photodetectors 13a and 13b that convert the diffracted light from the third area 2c of the hologram 2 into electric signals are 13aS and 13bS, and the third focus error signal F3 is
F3 = 13aS-13bS (5)
When the focus is not coincident with the information recording layer, the output values of the focus error signals F1, F2, and F3 correspond to the amount of the focal position shift.

  Therefore, in order to always keep the focal position coincident with the information recording layer, the two-element objective lens 9 is moved in the direction of the optical axis OZ so that the outputs of the first focus error signals F1 to F2 and F3 are always zero. Just fine.

  The method of detecting the defocus by the method as described above is generally called a knife edge method. Here, the focal position deviation indicates the amount of separation between the focal point where the light beam passing through the two-element objective lens 9 from the semiconductor laser 1 side is converged and the position of the information recording layer of the optical disk 6.

Normally, the detection of the focal position shift signal FES is performed using the entire effective diameter of the light beam.
FES = F1 + F2 + F3 (6)
It becomes. However, in that case, the following problem occurs.

  In the two-element objective lens 9 serving as the condensing optical system, spherical aberration occurs due to a change in the thickness of the cover glass 6a of the optical disk 6, and the like. Considering the detection of the focus position shift at this time, the offset A as shown in FIG. 4 occurs in the signal FES of the focus position shift. Therefore, even if the detected focus position shift signal FES outputs 0, the light beam does not coincide with the best image point O on the information recording layer, and there is a possibility that information cannot be recorded and reproduced. Here, the best image point O is a position of an image point at which the beam diameter of the light beam becomes minimum.

  Here, a description will be given below of the focal position shift between the inner peripheral portion and the outer peripheral portion of the light beam when spherical aberration occurs in the optical system of the optical pickup device 10.

  First, when no spherical aberration occurs in the light beam, as shown in FIG. 5A, the light beam is focused on one point (focal point) on the optical axis OZ.

  On the other hand, when spherical aberration occurs in the light beam, as shown in FIG. 5B, a focal point A is formed at a position farther than the best image point O on the optical axis OZ at the outer periphery of the lens, and the inner periphery of the lens is formed. On the side closer to the optical axis OZ of the portion, the focal point B is formed at a position closer to the best image point O.

  Therefore, as shown in FIG. 5B, when a spherical aberration occurs in the light beam, the focal position shift amount is the distance a from the best image point O to the focus A or the distance a from the best image point O to the focus B. It is indicated by the distance b.

  Therefore, when spherical aberration occurs in the light beam, it is necessary to condense the separated light beam to the best image point O in order to accurately detect the focal position deviation without causing an offset.

  Therefore, detection of defocus where no offset occurs is considered from the wavefront of the light beam. FIG. 6 shows the wavefront aberration when spherical aberration occurs, as shown in FIG. 5B. In addition, when spherical aberration occurs, the wavefront 17 when the light beam is the best image point on the information recording layer of the optical disk 6 is indicated by a curve symmetric about the optical axis OZ.

  Here, the slopes of the wavefronts of the regions 18a and 18b substantially coincide with the slopes of the ideal wavefront 16. Therefore, the light beams included in the regions 18a and 18b travel in the same direction as the ideal wavefront 16 and are focused at almost the same position as when there is no spherical aberration. Therefore, when the light beams included in the regions 18a and 18b are used, it is possible to detect the focal position shift while suppressing the influence of the spherical aberration.

  That is, as shown in FIG. 7, the focal position on the ideal wavefront 16 having no spherical aberration coincides with the extreme focal positions on the regions 18a and 18b that are the boundaries between the wavefronts 17 on which spherical aberration has occurred. . Therefore, if the focal position is detected by the extreme value of the wavefront 17 where the spherical aberration has occurred and the light beams corresponding to the regions 18a and 18b near the extreme value, the focal position (the best image point O) in the optical pickup device 10 is detected. ) Is shifted, the focal position of the extremum is shifted in the same manner, so that it is possible to detect the focal position shift while suppressing the influence of spherical aberration.

Next, the positions of the regions 18a and 18b will be searched. The regions 18a and 18b are the extremum of the wavefront 17 and the surrounding region. Generally, in the analysis of the aberration of the wavefront, the shape of the wavefront is fitted to the Zernike polynomial by the least square approximation, and the third-order aberration is obtained from the coefficient of the polynomial. When the wavefront 17 of FIG. 6 is fitted by the least squares approximation using the Zernike polynomial, the term of 6q ⁴ −6q ² +1 (q is the distance from the beam center normalized by the effective beam diameter) becomes dominant. Then, when the extremum of the wavefront 17 is obtained, the positions of the regions 18a and 18b are known. In FIG. 6, the distance r1 from the optical axis OZ to the extremum of the wavefront 17 is approximately equal to the effective beam diameter r of the light beam by the following equation (7). The relationship is as shown.

r1 = 0.7r (7)
That is, the position of about 70% of the beam effective diameter becomes the extreme value of the wavefront 17.

  FIG. 8 shows the offset amount of the focus position shift signal when the focus shift is detected in the region of 60 to 85% of the effective diameter of the light beam when the thickness of the cover glass 6a of the optical disk 6 changes and spherical aberration occurs. A calculation result of the offset amount of the focus position shift signal detected using the entire effective diameter of the light beam as in the above-described equation (6) is shown.

  Here, if the amount of spherical aberration increases, the best image point is shifted toward the focal position of the outer peripheral portion of the light beam. Therefore, the region set by calculation is 70% or less of the effective diameter of the light beam. The area occupied is larger than the area.

  Further, in many cases, the numerical aperture of the two-element objective lens 9 is calculated as 0.85, and even if the cover glass thickness is changed from 100 μm to ± 20 μm to generate spherical aberration, the focal point is in a region around 70% of the effective diameter of the light beam. When the shift is detected, it can be seen that almost no offset occurs in the focus position shift signal FES.

  In the optical pickup device 10, the detection of the focal position shift in the light beam near the effective beam diameter of 70% is performed by setting the diameter of the second arc E2 of the hologram 2 to be larger than 70% of the effective beam diameter. By making the arc E3 smaller than 70% of the effective diameter of the light beam, the second region 2b is designed to be a region near 70% of the effective diameter of the light beam, and the second light beam guided from the second region 2b is used. Just do it. At this time, the electric signal F2 becomes the defocus signal FES of the two-element objective lens 9.

The remaining first and third focus error signals F1 and F3 can be used for detecting spherical aberration. Since an offset corresponding to the magnitude of the spherical aberration occurs in the first focus error signal F1 and the third focus error signal, the spherical aberration can be detected from the values. Therefore, the spherical aberration error signal SAES is
SAES = F1 (8)
No,
SAES = F3 (9)
Generated by

By the way, when the focal position shift occurs, the spherical aberration error signal SAES changes, so that the spherical aberration cannot be detected accurately. However, when focus servo is applied, equations (8) and (9) may be used as SAES generation equations. However, if spherical aberration is to be detected while minimizing the influence of focal position deviation, the spherical aberration error signal SAES is expressed as
SAES = F3−F2 × K1 (K1 is a coefficient) (10)
Or
SAES = F1−F2 × K2 (K2 is a coefficient) (11)
And give it. At this time, the coefficients K1 and K2 are determined so that the change in SAES becomes small even if the focal position shifts. When the focus servo is considered, the coefficients K1 and K2 may be determined so that the value of SAES is stable in a range where the focal position shift is small.

Further, the spherical aberration error signal SAES is calculated as follows: SAES = F1−F3 × K3 (K3 is a coefficient) (12)
You may ask for it. Here, the coefficient K3 is a coefficient for correcting an output difference between the second focus error signal F2 and the third focus error signal F3.

  In the present embodiment, the hologram 2 is used as a means for guiding the light beam reflected from the information recording layer of the optical disc 6 to the detection device 7, but the present invention is not limited to this. However, it is preferable to use a hologram in order to reduce the size of the apparatus.

  Further, in the present embodiment, the knife edge method is used to detect the focal position shift, but instead of the hologram 2 shown in FIG. 1, a hologram 20 having a divided pattern as shown in FIG. Similarly, the beam size method of detecting the change in the beam size of the ± 1st-order light diffracted in the region 20a and detecting the focus position shift can also detect the focus position shift without occurrence of the offset.

In the optical pickup device shown in FIG. 9, the focal position shift signal FES uses outputs 30aS to 30cS and 31aS to 31cS from the photodetectors 30a to 30c forming the detecting device 30 and 31a to 31c forming the detecting device 31. Thus, it is generated using the following equation (13).
FES = {30bS- (30aS + 30cS)}-{31bS- (31aS + 31cS)} (13)
Further, the diffracted light from the area 20b on the outer periphery of the area 20a of the hologram 20 and the diffracted light from the area 20c on the inner periphery of the area 20a are detected in the same manner as the method of detecting the focal position shift from the diffracted light from the area 20a. It can be used for detecting spherical aberration by detecting the focal position shift, but is omitted here.

  In the present embodiment, as shown in FIG. 1, the hologram 2 is divided by a circle or a circular arc.

  However, in an actual optical pickup device, the two-element objective lens 9 is moved in the radial direction (radial direction) of the optical disc 6 so that the light is always focused on the track formed on the recording information layer of the optical disc 6. Control for converging light on the top, so-called tracking control, is performed.

  This is not a problem when the hologram 2 and the two-element objective lens 9 are integrally manufactured. However, when each member is separated and mounted on the optical pickup device, the center of the light beam is controlled by tracking control. Situation that does not coincide with the center of

  At this time, if the shape of the hologram 2 is as shown in FIG. 1, a part of the light beam that should have been diffracted in each region of the hologram 2 is diffracted in different regions, and the center of the light beam is The electric signal from each photodetector changes depending on whether or not there is a deviation from the center of the hologram 2. Therefore, each error signal generated for controlling the optical pickup device changes.

  Therefore, it is conceivable to use a hologram 21 as shown in FIG. 10 instead of the hologram 2 shown in FIG. The hologram 21 has regions 21a and 21b divided by a straight line parallel to the radial direction, and ± 1st order light from these regions 21a and 21b is respectively condensed at the same point to detect a change in beam size. The shift of the focal position of the two-element objective lens as the light collecting optical system is detected.

In this case, the regions 21a and 21b are designed so as to diffract the light beam in a region 70% of the effective diameter of the light beam and in the vicinity thereof. The focal position shift signal FES at this time is obtained by the following equation (3) using the output signals 32aS to 32cS and 33aS to 33cS from the photodetectors 32a to 32c constituting the detecting device 32 and the 33a to 33c constituting the detecting device 33. 14).
FES = {32bS- (32aS + 32cS)}-{33bS- (33aS + 33cS)} (14)
If the dividing line for dividing the area in the hologram 21 is as shown in FIG. 10, the two-element objective lens 9 moves in the radial direction of the optical disc 6 for tracking control, so that the area where the light beam is diffracted changes. Since there is no light, the light in the same area in the light beam is always focused on the same light receiving portion. Therefore, a signal such as a focal position shift does not change due to the tracking control.

  In addition, when the optical pickup device of the present embodiment is used, even if the thickness of the cover glass 6a of the optical disk 6 changes over a wide range, it is possible to accurately detect the defocus, so that the recording having a plurality of information recording layers is possible. It is possible to match the focus to each information recording layer on the medium.

  In the present embodiment, the objective lens is a two-element objective lens composed of two lenses, the first lens element 4 and the second lens element 5. However, in order to simplify the assembly of the apparatus, one objective lens is used. The objective lens 9 may be constituted by a lens.

  Further, in the present embodiment, the correction of the focal position deviation is performed by changing the distance between the two-element objective lens 9 and the optical disk 6 based on the focal position deviation signal FES detected by the above-described detection method. The spherical aberration can be corrected by changing the distance between the first lens element 4 and the second lens element 5 constituting the two-element objective lens 9, but is not limited to this. For example, the distance between the semiconductor laser 1 and the collimating lens 3 may be adjusted by moving the collimating lens 3.

  Further, a spherical aberration correcting mechanism may be inserted between the two-element objective lens 9 and the collimating lens 3. The spherical aberration correction mechanism constitutes an optical system that generates spherical aberration when a light beam passes through the spherical aberration correction mechanism.

  For example, an afocal optical system combining a convex lens having a positive power and a concave lens having a negative power may be used as the spherical aberration correction mechanism. By adjusting the distance between the two lenses, spherical aberration can be generated. Further, as another configuration of the spherical aberration correcting mechanism, an afocal optical system combining two convex lenses having positive power may be used. Further, a liquid crystal panel may be considered as a mechanism for correcting spherical aberration by generating spherical aberration.

  In the above spherical aberration correction method, the spherical aberration is corrected by generating a spherical aberration that cancels out the spherical aberration generated in the focusing optical system. However, for example, when finding the position of the two-lens correction mechanism and the optimum spherical aberration correction lens, the spherical aberration may be increased by moving the lens in a direction to increase the spherical aberration. At that time, in the method of detecting the focal position deviation using the light beam in the entire effective diameter of the light beam, an offset occurs in the focal position deviation signal, so that the diameter of the light beam is not minimized on the recording layer, and the recording or the recording is not performed. The reproduced signal is degraded.

  However, if the focal position deviation is detected by the above-described detection method, no offset occurs in the focal position deviation signal FES even if the amount of spherical aberration fluctuates to a large extent. Can be matched with each information recording layer of the optical disc 6 having the above information recording layer, and the recording or reproduction signal does not deteriorate.

FIG. 1 is a schematic configuration diagram of an optical pickup device of the present invention. FIG. 2 is a schematic configuration diagram of an optical recording / reproducing device including the optical pickup device shown in FIG. 1. FIG. 2 is an explanatory diagram illustrating details of a detection device of the optical pickup device illustrated in FIG. 1. 9 is a graph illustrating a relationship between a focus error signal FES and a focus position shift amount when spherical aberration occurs in the light collecting optical system. (A) is an explanatory view showing a focal position of a light beam in a lens having no spherical aberration, and (b) is an explanatory view showing a focal position of a light beam in a lens having spherical aberration. FIG. 4 is a conceptual diagram illustrating wavefront aberration when spherical aberration occurs. FIG. 7 is an explanatory diagram showing a state of a focal position when there is no spherical aberration and a focal position at an extreme value of a wavefront when there is spherical aberration. 11 is a graph showing a relationship between an offset amount and a cover glass thickness error when a focus error signal FES is detected by changing an effective diameter of a light beam. FIG. 11 is a schematic configuration diagram of another optical pickup device of the present invention. FIG. 13 is a schematic configuration diagram of still another optical pickup device of the present invention.

Explanation of reference numerals

Reference Signs List 1 semiconductor laser 2 hologram (light beam separation means)
2a area 2b area 2c area 2d area 2e area 4 First lens element (objective lens)
5. Lens second element (objective lens)
6 Optical disk (optical recording medium)
7 Detector 8 Detector 9 Two-element objective lens (condensing optical system)
16 Ideal wavefront 17 Wavefront (curve)
18a area (near extremum)
18b area (near extremum)
20 Hologram (light beam separation means)
20a area 20b area 20c area 21 hologram (light beam separation means)
21a Area 21b Area CL1 First straight line CL2 Second straight line E2 Second circular arc E3 Third circular arc F1 First focus error signal F2 Second focus error signal F3 Third focus error signal O Best image point OZ Optical axis SAES Spherical aberration error signal r Light beam effective diameter

Claims (7)

The light beam reflected from the information recording layer of the optical recording medium and passed through the condensing optical system including the objective lens,
A first circle or arc having a diameter larger than 70% of the effective light beam diameter defined by the aperture of the objective lens, and 70% of the effective light beam diameter defined by the aperture of the objective lens; Light in a region surrounded by a second circle or a circular arc having a diameter smaller than the diameter and having a larger proportion of the region of 70% or more of the effective diameter of the light beam than the region of 70% or less of the effective diameter of the light beam. Split the beam,
The separated light beam is electrically converted to generate a first focus error signal, and the first focus error signal is used as a focus position shift signal indicating a focus position shift of the focusing optical system. Position shift detection method. The first focus error signal is F1, the second focus error signal obtained by detecting the focus position shift of the light beam obtained by passing through the area located inside the second circle or arc is F2, and the second focus error signal is F2. Assuming that a third focus error signal obtained by detecting a focus position shift of a light beam obtained by passing through a region located outside of the circle or the arc 1 is F3,
A spherical aberration error signal SAES indicating a spherical aberration generated in the light-collecting optical system is:
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
Or
SAES = F3-F2 × K3 (K3 is a coefficient)
The focus position deviation detecting method according to claim 1, wherein the focal position deviation detecting method is obtained by the following expression. A light source,
A condensing optical system including an objective lens for condensing a light beam emitted from the light source on an optical recording medium,
A center of the optical axis of a light beam reflected from the information recording layer of the optical recording medium and having passed through the condensing optical system, and a diameter larger than 70% of the effective diameter of the light beam defined by the aperture of the objective lens. And a second circle or arc having a diameter smaller than 70% of the effective diameter of the light beam defined by the aperture of the objective lens. Light beam separating means for separating a light beam in a region where the area of 70% or more of the diameter occupies a larger area than the region of 70% or less of the effective diameter of the light beam;
First signal generation means for electrically converting the light beam separated by the light beam separation means to generate a first focus error signal,
An optical pickup device, wherein the first focus error signal is a focus error signal indicating a shift of a focal position of the focusing optical system. A second signal generating unit configured to detect a focal position shift of a light beam obtained by passing through a region located inside the second circle or the arc of the light beam separating unit and generate a second focus error signal; ,
A third signal generation unit configured to detect a focal position shift of a light beam obtained by passing through a region located outside the first circle or the arc of the light beam separation unit and generate a third focus error signal; Has,
When the first focus error signal is F1, the second focus error signal is F2, and the third focus error signal is F3,
A spherical aberration error signal SAES indicating a spherical aberration generated in the condensing optical system,
SAES = F2−F1 × K1 (K1 is a coefficient)
Or
SAES = F3-F1 × K2 (K2 is a coefficient)
Or
SAES = F3-F2 × K3 (K3 is a coefficient)
4. The optical pickup device according to claim 3, further comprising: a spherical aberration detecting unit that is determined by the following equation. When the optical recording medium has a plurality of information recording layers,
5. The optical pickup device according to claim 3, wherein the focus position shift detecting means detects a focus position shift in each information recording layer. A focal position deviation correcting unit that corrects a focal position deviation of the light collecting optical system based on a detection result detected by the focal position deviation detecting unit;
A spherical aberration corrector that corrects spherical aberration generated in the light-collecting optical system based on a detection result detected by the spherical aberration detector.
6. The spherical aberration corrector according to claim 3, wherein the spherical aberration corrector corrects the spherical aberration of the light collecting optical system in a state where the focal position shift of the light collecting optical system is corrected by the focal position shift corrector. The optical pickup device according to claim 1.   The optical pickup device according to any one of claims 3 to 6, wherein the light beam separating means is a hologram.

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