JP2004281050A - Optical head device and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head device which condenses a light beam on information media (optical disks) different in thickness up to the limit of diffraction by using a hologram lens with the sufficient intensity of transmitted light. <P>SOLUTION: In designing an objective lens, the passage region of light which is incident upon an objective lens used for an optical head is separated into at least a first and second regions according to the distance of the passage region from the optical axis of the incident light. The designing is made so that the light passing through the first region converges on a disk with a transparent substrate of t2 in thickness and the light passing through the second region converges on an optical disk with a transparent substrate of t1+t2 (t2<t1) in thickness by providing a concave-and-convex shaped ring zone. The ring zone is concentrically circular about the optical axis of the objective lens, and the ring zone is integrally formed with the surface of large curvature, that is, surface of a small radius of curvature of the objective lens, thereby restraining the aberration occurring when the ring zone tilts to the optical axis. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical head device for recording / reproducing or erasing information stored on an optical medium or a magneto-optical medium (information medium) such as an optical disk or an optical card.

  Optical memory technology using an optical disk having a pit-shaped pattern as a high-density, large-capacity storage medium has been put to practical use while expanding its applications to digital audio disks, video disks, document file disks, and data files. . The mechanism by which recording and reproduction of information on and from an optical disk via a minutely focused light beam is successfully performed with high reliability depends solely on the optical system. The basic functions of the optical head device, which is the main part of the optical system, are broadly divided into light-collecting ability to form a diffraction-limited small spot, focus control and tracking control of the optical system, and detection of a pit signal. You. These are manifested by combinations of various optical systems and photoelectric conversion detection systems depending on the purpose and application.

  In recent years, with the progress of optical system design technology and the shortening of the wavelength of a semiconductor laser as a light source, the development of an optical disc having a higher-density storage capacity than before has been progressing. As an approach for increasing the density, it has been considered to increase the numerical aperture (NA) on the optical disc side of a condensing optical system that narrows a light beam onto an optical disc. At this time, a problem is an increase in the amount of aberration generated due to the tilt (so-called tilt) of the optical axis. Increasing the numerical aperture NA increases the amount of aberration generated with respect to tilt. To prevent this, the thickness (base material thickness) of the substrate of the optical disk may be reduced. For example, to obtain the same amount of tilt tolerance as when NA = 0.5 and the substrate thickness t1 = 1.2 mm, the substrate thickness t2 = 0.6 mm when NA = 0.6. For the above reasons, it is desirable to reduce the thickness of the substrate in a high-density optical disk. For this reason, it is considered that the substrate of the next-generation high-density optical disk will be thinner than many conventional optical disks including a compact disk (CD) already on the market. Naturally, an optical disk device that can record and reproduce both conventional optical disks and next-generation high-density optical disks is required. For that purpose, an optical head device having a condensing optical system capable of condensing a light beam to a diffraction limit on an optical disk having a different substrate thickness is required.

The present inventors have already proposed an optical head that can be used for a plurality of optical disks having different substrate thicknesses (Japanese Patent Laid-Open No. 7-98431). In an optical head, a light beam from a light source is converted into parallel light, focused on a minute spot by an objective lens, and irradiated onto an optical disk. The reflected light from the optical disk follows the original optical path in reverse, but is reflected by the beam splitter and detected by the photodetector. Here, in order to use the optical head for a plurality of optical disks having different substrate thicknesses, a bifocal lens combining an objective lens and a hologram lens for diffracting a part of the incident light is used. A condensed spot condensed to the diffraction limit is formed on each. Since the hologram lens diffracts a part of the incident light, it has, for example, a concentric lattice pattern, and the transmitted light (zero-order diffracted light) also has a sufficient intensity. The light diffracted by the hologram lens and the light that is not diffracted are collected at different focal positions on the optical axis. Therefore, minute spots can be formed on substrates having different thicknesses. Here, since the hologram lens has a lens function, the positions of the two focal points in the optical axis direction are different, and when recording and reproducing information at one focal point, the light beam having the other focal point as the focal point is It spreads widely, has low light intensity, and does not affect recording and reproduction.
JP-A-7-98431 JP-A-61-189504 JP-A-63-241735 JP-A-58-85944 JP-A-62-67737

The optical head device using the above-mentioned bifocal lens has points to be improved and developed. For example, the bifocal lens is configured by combining an objective lens and a hologram lens, but various configurations are conceivable. Particularly, in order to reduce the size of the optical head device, it is desirable to use a semiconductor laser as a light source, but there is a problem caused by using a semiconductor laser. As shown in FIG. 1, the semiconductor laser emits a light beam from a light emitting point 2002 near an end face of the active layer 2001. Here, the far-field image of the light beam has a wider spread angle θ _{Y in} the Y direction orthogonal to the active layer 2001 than in the spread direction θ _{X in the} X direction parallel to the active layer 2001. FIG. 2 shows a light intensity distribution 2003 in the X direction (a) and the Y direction (b) of a light beam (when the light beam diameter is 4 mm) emitted from the semiconductor laser. As described above, the distribution is significantly different in both directions. When this light beam is made incident on the above-mentioned hologram lens that diffracts a part of the incident light, the light intensity of the outer peripheral portion is more inner in the X direction (a) and the Y direction (b), as indicated by the hatched portions. Stronger than the periphery. Since the divergence angle in the Y direction is wide, the light intensity at the outer peripheral portion in the Y direction is particularly stronger than the light intensity at the inner peripheral portion.

  Here, the side lobe will be described. FIG. 3 shows the distribution of the light intensity distribution in the X direction (a) and the Y direction (b) of the condensed spot on the information medium having a small substrate thickness when a hologram lens that diffracts a part of the incident light is used. It shows a calculation result. Here, the light intensity was standardized with the maximum value of the main lobe 380 being 100. The main lobe 380 is an amount of light necessary for recording and reproduction, and the side lobe 381 is an unnecessary amount of light that causes deterioration of a recording pit shape and a reproduction signal. When the maximum value of the light intensity of the main lobe 380 is normalized to 100, the light intensity of the side lobe 381 is about 1% in the X direction, which is sufficiently low, but slightly higher in the Y direction, about 4%. Even if the height of the side lobe is about 4%, it is possible to read the information signal sufficiently. However, in order to read the information signal more stably against disturbances such as vibration and temperature change, the side lobe needs to be increased. It is desirable to reduce the influence by a method such as lowering.

A first object of the present invention is to make it possible to focus a light beam up to a diffraction limit on an information medium (optical disk) having a different substrate thickness by using a hologram lens having a sufficient intensity of transmitted light. An object of the present invention is to provide an optical head device capable of obtaining a more stable information signal by alleviating the influence of an increase in the light intensity at the outer peripheral portion.
A second object of the present invention is to improve a light beam to a diffraction limit on an information medium (optical disc) having a different substrate thickness by using a hologram lens having a sufficient intensity of transmitted light. An object of the present invention is to provide an optical head device.

  A first optical head device according to the present invention includes a radiation light source, an objective lens for condensing light emitted from the radiation light source on an optical disc through different substrate thicknesses, and receiving light reflected from the optical disc and returned. The optical detector having a different substrate thickness to reproduce information by receiving light reflected from the optical disk and returned by the optical detector. Here, the objective lens divides the objective lens surface into at least a first and a second plurality of regions according to a distance of an incident light passing area from the optical axis of the incident light, and divides the first area. A first hologram is formed in an outer peripheral portion, a first hologram is formed in the first region, and a first-order diffracted light obtained by diffracting light passing through the first region by the first hologram is an optical disk having a transparent substrate thickness t2. And the second region is provided with a second hologram composed of a concavo-convex annular zone in the second region, so that light passing through the second region has a thickness of the transparent substrate of either t1 or t2. The convergence with respect to the optical disk, t2 <t1, the orbicular zone has a concentric shape centered on the optical axis of the objective lens, and the orbicular zone has a larger curvature of the objective lens. In other words, integrated on the surface with the smaller radius of curvature It characterized by suppressing the aberrations generated when tilted with respect to the optical axis by.

  The first optical head device preferably records and reproduces both an optical disk corresponding to a transparent substrate thickness t1 and an optical disk corresponding to a transparent substrate thickness t2 different from t1.

  A second optical head device according to the present invention includes a radiation light source, an objective lens that receives a light beam emitted from the radiation light source and converges to a minute spot on an optical disc, and receives a light beam reflected and diffracted by the optical disc. And a hologram provided in the optical path from the radiation light source to the objective lens and partially having a diffraction grating portion. Here, the objective lens divides the objective lens surface into at least a first and a second plurality of regions according to a distance of an incident light passing area from the optical axis of the incident light, and divides the first area. A first hologram is formed in an outer peripheral portion, a first hologram is formed in the first region, and a first-order diffracted light obtained by diffracting light passing through the first region by the first hologram is an optical disk having a transparent substrate thickness t2. And the second region is provided with a second hologram composed of a concavo-convex annular zone in the second region, so that light passing through the second region has a thickness of the transparent substrate of either t1 or t2. The convergence with respect to the optical disk, t2 <t1, the orbicular zone has a concentric shape centered on the optical axis of the objective lens, and the orbicular zone has a larger curvature of the objective lens. In other words, integrated on the surface with the smaller radius of curvature Suppressing aberration that occurs when tilted with respect to the optical axis by. In the hologram, a portion near the optical axis connecting the radiation light source and the center of the objective lens is a transmission portion without the diffraction grating portion, and the diffraction grating portion is formed in a region away from the optical axis. Then, of the diffracted light diffracted from the diffraction grating portion, the diffracted light diffracted in a direction approaching the optical axis is condensed on the optical disk by the objective lens, and the light reflected from the optical disk and returned is detected by the photodetector. , A tracking error signal is detected.

  In the second optical head device, preferably, the light beam emitted from the radiation light source is perpendicular to the radiation direction, and has a different rate of change in light intensity in two directions perpendicular to each other. When the direction in which the rate of change of the light intensity is the slowest is defined as the Y direction and the direction perpendicular to both the Y direction and the radiation direction is defined as the X direction, the area where the diffraction grating portion is formed is defined as the Y direction. It is provided in a direction away from the optical axis.

  In the second optical head device, preferably, the diffraction grating portion is blazed in a direction in which the intensity of the first-order diffracted light diffracted in a direction approaching the optical axis increases.

  Preferably, the second optical head device further includes a collimator lens for converting a light beam emitted from the radiation light source into parallel light between an optical path from the radiation light source to the objective lens, and a light intensity correction unit. , Formed integrally with the surface of the collimating lens.

  In the first or second optical head device, preferably, the objective lens is a first region of the objective lens designed to focus light only on an optical disk having a specific transparent substrate thickness t2. The numerical aperture NA1 for the transparent substrate thickness t1 is made smaller (NA1 <NA2) than the numerical aperture NA2 for the transparent substrate thickness t2 by setting the outermost peripheral area farthest from the optical axis.

  In the first or second optical head device, preferably, the + 1st-order diffraction efficiency of the first hologram is set to approximately 100%.

  In the first or second optical head device, preferably, a tracking error signal is generated by condensing light emitted from a radiation light source on an optical disk and receiving light reflected from the optical disk and returned by a photodetector. Detects and reproduces information from the optical disk.

  Here, the “objective lens” includes not only the “objective lens” in a narrow sense but also a broadly complex lens that converges a light beam to a minute spot on an information medium. In this optical head device, by combining the objective lens with the hologram lens that diffracts a part of the incident light in the objective lens, the diffraction limit is set on an optical disk (information medium) having different substrate thicknesses (t1 and t2). It is possible to realize a bifocal lens capable of forming a condensed spot that condenses light up to the maximum. Using this bifocal lens, a small, lightweight, low-cost optical head device with a small number of components can be used to perform recording and reproduction of optical disks having different substrate thicknesses with one optical head device.

  An optical disc device according to the present invention uses any one of the optical head devices described above to condense light emitted from a radiation light source onto an information medium, and receive light reflected from the information medium and returned by a photodetector. Thus, a tracking error signal is detected, and information is reproduced from the information medium.

According to the present invention, the following effects can be obtained.
(1) By combining a hologram lens that diffracts a part of the incident light and an objective lens, light condensed to the diffraction limit on optical disks (information media) having different substrate thicknesses (t1 and t2). A bifocal lens capable of forming a spot can be realized. Using this bifocal lens, a small, lightweight, low-cost optical head device with a small number of parts can be used for recording and reproducing optical discs having different substrate thicknesses. And two optical head devices.
(2) By providing a light intensity correction means such as a correction hologram or a correction filter, the side lobe of the light beam (condensed spot) on the information medium is further reduced, and a reproduced signal having excellent characteristics can be obtained. it can.
(3) When a semiconductor laser is used as a light source by increasing the thickness of the portion having high zero-order diffraction efficiency or transmittance of the light intensity correcting means such as a correction hologram or a correction filter, thereby increasing the optical path length, The adverse effects of point difference can be eliminated or reduced.
(4) In combination with a light intensity correcting means such as a correction hologram or a correction filter, a large (preferably φ1 mm or more) photodetector is provided around the photodetector for servo signal detection to receive light on the outer periphery, By using the sum of the two photodetectors as an information signal, the S / N ratio can be further improved, and the frequency characteristics can be further improved.
(5) Light intensity correction means such as a correction hologram or a correction filter is formed on the surface of the objective lens or the collimator lens, so that the number of components can be reduced and the cost can be further reduced.
(6) When the hologram lens is formed on the surface of the objective lens, the surfaces of the lattice plane of the lattice pattern and the plane without the lattice pattern are continuously connected, and the average plane of the lattice pattern and the lattice plane are not connected. By designing the surface with and without the surface to be able to condense the light beam to the diffraction limit, it is possible to obtain good light condensing characteristics.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 4 shows an optical head device according to the first embodiment of the present invention. The features of this optical head device are that a bifocal lens is constituted by the objective lens 4 and the hologram lens 107, and that a correction hologram 1221 for reducing the amount of light in the outer peripheral portion is provided. In this optical head device, the light beam 3 emitted from the radiation light source 2 such as a semiconductor laser is converted into substantially parallel light by the collimator lens 122, passes through the correction hologram 1221, and the light intensity at the outer peripheral portion is reduced. The light beam further passes through the polarizing beam splitter 42 and is converted into circularly polarized light by the quarter-wave plate 15. Next, the light beam enters the hologram lens 107 and the objective lens 4 and is focused on the thick information medium 5 or the thin information medium 51 located at different focal positions. The information medium 5 is an optical disk having a substrate thickness t1 = 1.2 mm, and the information medium 51 is an optical disk having a substrate thickness t2 = 0.6 mm. Here, the “thickness of the substrate” refers to the thickness from the surface on which the light beam enters the information medium to the information recording surface. In addition, although the term “light collection” is used here, “light collection” is defined as “to converge divergent light or parallel light to a diffraction-limited small spot”.

  As schematically shown in FIGS. 5 and 6, the hologram lens 107 is formed on a substrate 9 which is transparent to the light beam 3, and has a ring-shaped grid pattern 107a at the center and a grid pattern around it. And the region 107b without the region. The lattice pattern 107a is concentric, and its center, that is, the optical axis coincides with the objective lens 4 within an assembly error.

  The diffraction efficiency of the + 1st-order diffracted light of the hologram lens 107 is less than 100%, and the hologram lens 107 is designed so that the transmitted light (0th-order diffracted light) 61a of the light beam 3a also has a sufficient intensity. This is because, when the hologram lens 107 is manufactured in an uneven shape as shown in FIG. 4, for example, the height h of the (relief type) unevenness is smaller such that h <λ / (n−1). It can be easily realized by making the amplitude of the phase change given to the light beam by the grating unit 107a smaller than 2π. Here, λ is the wavelength of the light beam 3 and n is the refractive index of the transparent substrate 9. As described above, by making the transmitted light have a sufficient intensity at any position of the hologram lens 107, the side lobe (see FIG. 7) of the condensed beam formed by the transmitted light on the information medium 51 is suppressed. Has the effect that can.

  The hologram lens 107 is composed of a lattice pattern 107a and a region 107b having no lattice pattern. The phase of the zero-order diffracted light (transmitted light) of the grating pattern 107a is an average value of the amount of phase modulation given by the grating pattern 107a. On the other hand, it is desirable to improve the light-collecting performance by adjusting the phases of the transmitted light in the region 107b without the lattice pattern to the same degree. Therefore, for example, when the hologram lens 107 has a lattice pattern 107a of a relief type, the height of the surface of the region 107b without the lattice pattern is adjusted to the level of the average of the irregularities of the lattice pattern portion as shown in FIG.

  The light beam reflected by the information medium 5 or 51 reverses the original optical path. The transmitted light 61 again passes through the hologram lens 107 as indicated by the solid line, and the + 1st-order diffracted light 64 is diffracted again as a + 1st-order diffracted light by the hologram lens 107 as indicated by the broken line. The light passes through the same optical path as that after passing through the light source, and is reflected by the polarization beam splitter. The reflected light is condensed by a converging lens 121, and the wavefront is converted by a wavefront converting means such as a cylindrical lens 131 so that a servo signal such as a focus error signal or a tracking error signal can be obtained. Incident on the container 71. By calculating the output of the photodetector 71, a servo signal (a focus error signal and a tracking error signal) and an information signal can be obtained.

  The hologram lens 107 can be blazed as shown in FIG. 4, for example, to increase the sum of the amounts of transmitted light and + 1st-order diffracted light that form a bifocal light beam, as described later. There is an effect that utilization efficiency can be increased.

  As shown in FIG. 8A, when the light beam 61 transmitted through the hologram lens 107 without being diffracted is incident, the objective lens 4 has a numerical aperture NA of 0.6 or more and the thickness t2 of the substrate 37. It is designed to be able to form a diffraction-limited condensed spot 38a on an optical disk 51 having a small thickness. Further, in this case, the lattice pattern 107a of the hologram lens 107 is formed only within a diameter smaller than the aperture determined by the objective lens 4. Therefore, no diffraction occurs in the portion 107b of the hologram lens 107 where the lattice pattern is not formed, and the light amount of the high NA focused spot 38a increases.

  On the other hand, FIG. 8B shows that the focused spot 38b can be focused to the diffraction limit on the thick (thickness t1) information medium 5 of the substrate 37 with a low NA. The + 1st-order diffracted light 64 diffracted by the hologram lens 107 is collected on the information medium 5 by the objective lens 4. Here, the + 1st-order diffracted light 64 has been subjected to aberration correction so as to be narrowed down to the diffraction limit through the substrate 37 having a thickness t1. The design method of the hologram lens 107 having such an aberration correcting function is as follows. For example, a spherical wave diverging from the condensing spot 38b transmits through the substrate 37 having a thickness t1, then transmits through the objective lens 4, and An interference pattern (a hologram lens lattice pattern 107a) between the light beam transmitted through the formed transparent substrate 9 and the light beam obtained by inverting the sign of the light beam 3 in FIG. 8B may be calculated. The hologram lens 107 can be easily manufactured by, for example, a computer generated hologram (CGH) method. As described above, by combining the hologram lens 107 that diffracts a part of the incident light and the objective lens 4, condensed spots condensed to the diffraction limit on optical discs having different substrate thicknesses (t1 and t2). Can be realized.

  Here, since the hologram lens 107 has a lens function, the positions of the two focal points in the optical axis direction are different, and when information is recorded / reproduced at one focal spot, light having the other focal point as a focal point is used. The beam spreads greatly, the light intensity is small, and does not affect recording and reproduction. For example, as shown in FIG. 8A, when the converging spot 38a is at the focal point position with respect to the information medium 51, the + 1st-order diffracted light 64 spreads greatly on the information recording surface of the information medium 51, and Does not affect playback. This is the same in the case of FIG.

  The difference between the two focal positions affects the recording / reproducing when the information beam is recorded / reproduced at one focal spot, the light beam having the other focal point as the focal point spreads widely and the light intensity is small. In order to avoid this, it is desirable that the thickness be as large as possible at 50 μm or more. Further, since the substrate thickness t1 of a compact disk (CD) or a laser disk (LD) is about 1.2 mm, and the substrate thickness t2 of a high-density optical disk is considered to be 0.4 mm to 0.8 mm, the objective lens Considering the movable range of the actuator that performs the focus servo operation, it is desirable that the difference between the two focal positions does not greatly exceed the difference between t1 and t2 of about 0.8 mm. Therefore, as shown in FIG. 8A, when shortening the focal length of the condensing spot 38a corresponding to a thin substrate with a high NA, the difference between the two focal positions is set to 50 μm or more and 1 mm or less.

  In the present embodiment, as shown in FIG. 4, when the light beam 61 that has passed through the hologram lens 107 without being diffracted is incident thereon, the objective lens 4 has a diffraction limit on a thin optical disc having a thin substrate (t2). It is designed to form a focused spot. Further, the grating pattern 107a of the hologram lens 107 of the present embodiment is formed only within a diameter smaller than the aperture determined by the objective lens 4. Therefore, no diffraction occurs in the portion of the hologram lens 107 where the grating pattern 107b is not formed. As described above, by combining the hologram lens 107 that diffracts a part of the incident light and the objective lens 4, condensed spots condensed to the diffraction limit on optical discs having different substrate thicknesses (t1 and t2). Can be realized.

  The transmitted light 61 and the + 1st-order diffracted light, which have passed through the hologram lens 107 and have been reflected by the information media 5 and 51 again, pass through the same optical path as when they first passed through the polarizing beam splitter 42, and then pass through the polarizing beam splitter 42. Then, the light is reflected by the converging lens 121. The servo signal is detected by the photodetector 71 using the collected light beam. Therefore, the condensing point 39 of the light beam reflected from the two focal points on the photodetector side coincides with a point conjugate with the emission point of the radiation light source 2. Therefore, a single servo signal detecting means and a single photodetector 71 can be used in common, and a small, lightweight, low-cost optical head device with a small number of components, but an optical disc device having a different substrate thickness. There is an effect that recording and reproduction can be performed by one optical head device.

  As a method of detecting a focus error signal, any method such as a spot size detection method (SSD method: Japanese Patent Application Laid-Open No. 2-185722), an astigmatism method, and a knife edge method can be used. As the tracking error signal, any method such as a push-pull method, a heterodyne method, and a three-beam method can be adopted.

  One of the features of the present invention is that an element such as the correction hologram 1221 or the correction filter 1225 that partially corrects the light intensity at the outer peripheral portion is used. Although light has intensity and phase, these elements are light quantity and phase modulation elements, which are provided in an optical system to change the light quantity and phase as desired, and reduce the light intensity at the outer peripheral portion. .

  FIG. 9 shows an optical system on the outward path from the radiation light source 2 to the objective lens 4. In the correction hologram 1221, for example, as shown in (b), a transmissive portion 1223 that totally transmits light is provided near the optical axis 3000, and a diffraction grating portion 1222 is provided far from the optical axis 3000. . Then, as shown in (a), in the outward path, all the light near the optical axis of the light beam 3 emitted from the radiation light source 2 is transmitted, and the light away from the optical axis is partially diffracted and transmitted. Decrease rate.

  Here, the phase of the zero-order diffracted light (transmitted light) of the diffraction grating unit 1222 is an average value of the amount of phase modulation given by the diffraction grating unit 1221. On the other hand, it is desirable to improve the light-collecting performance of the objective lens 4 by adjusting the phase of the transmitted light in the region 1223 having no lattice pattern to the same degree. Therefore, for example, as shown in FIG. 9B, when the hologram has a relief-type grating pattern, the transmission portion 1223 has an average level of the unevenness of the diffraction grating portion 1222 (average of the top and bottom portions). Adjust the surface height.

  However, when a semiconductor laser is used as the radiation light source 2 and the semiconductor laser has an astigmatic difference (see FIG. 1), as shown in FIG. 10, the surface of the central transmission part 1223a including the optical axis 3000 is used. Is higher than the average of the diffraction grating part 1222. In other words, the thickness of the transmission part 1223a is made larger than that of the diffraction grating part 1222, and the optical path length is made longer. Thereby, the adverse effect of the astigmatic difference of the semiconductor laser can be reduced, and the light-collecting characteristics of the optical system can be improved. When the semiconductor laser has an astigmatic difference, a condensing point in a plane in a direction in which the spread angle of the emitted light is small enters the active layer of the semiconductor laser. Therefore, if the converging positions in both directions are made to coincide with each other by making the convex lens action of the optical system larger in a direction perpendicular to the direction, that is, in a direction in which the spread angle of the emitted light is wide, the wavefront aberration can be reduced. As described later, it is desirable to make the Y direction in FIG. 10 coincide with the direction in which the divergence angle is wide, so it is desirable to have a convex lens function in this Y direction. This is the reason why the thickness of the transmission portion 1223a should be increased.

  Further, the diffracted light 1224 diffracted from the correction hologram 1221 becomes unnecessary stray light. Therefore, it is desirable to design so that the light does not enter the opening of the objective lens 4 or does not enter the photodetector 71 after being reflected by the information medium 5 or 51.

  In order to prevent the diffracted light 1224 from entering the opening of the objective lens 4, the grating pitch of the diffraction grating 1222 is set to 5 μm or less, preferably 2 μm or less, or as shown in FIG. After the diffraction grating 1222 is blazed in a direction in which the intensity of the diffracted light diffracted away from the optical axis increases, the grating pitch is set to 20 μm or less, preferably 12 μm or less.

  As another method, in order to design so that the diffracted light 1224 does not enter the photodetector 71 after being reflected by the information medium 5 or 51, the grating pitch of the diffraction grating portion is set to 30 μm or less, preferably 10 μm or less. .

  Conversely, it is also possible to condense the diffracted light on the information medium, receive the reflected light from the information medium with a photodetector, and obtain a tracking error signal by the so-called three-beam method from the output. . In this case, contrary to the above-described case, it is desirable to blaze the diffraction grating portion 1222 in a direction in which the intensity of the diffracted light diffracting in the direction approaching the optical axis increases.

  FIG. 11 shows an example in which the correction hologram 1221 is viewed from the direction of the optical axis 3000 in FIG. 9, that is, the Z direction. The circle representing the optical axis 3000 and the circle representing the effective beam diameter 1224 are virtually drawn for explanation, and are not actually formed on the correction hologram 1221. The coordinate axes are common to FIG. 1 and FIG. In order to reduce the amount of light in the direction in which the spread angle of the light beam is wide, that is, in the portion away from the optical axis in the Y direction in FIG. 1, the portion away from the optical axis in the Y direction in FIG. Is desirable.

  In addition, since the contour line of the light intensity of the far-field image (FFP) has an elliptical shape, the light amount is made effective by making the boundary between the diffraction grating portion 1222 and the transmission portion 1223 convex outward from the optical axis 3000. The effect that it can be used for is obtained.

  FIG. 12 shows an example of a change in the zero-order diffraction efficiency (transmittance) of the correction hologram 1221 in the Y-axis direction. The origin is the intersection of the optical axis 3000 and the correction hologram 1221. The 0th-order diffraction efficiency is reduced toward the outer periphery. By providing the correction hologram 1221, the far-field image of the light beam that has passed through the hologram lens 107 is as shown in FIG.

  FIG. 13 shows the distribution of light intensity in the X direction (a) and the Y direction (b). By reducing the light intensity in the Y direction by the correction hologram 1221, the light intensity distribution (b) in the Y direction is different from that in FIG. That is, the light intensity at the outer periphery in the Y direction is lower than that at the inner periphery.

  In order to make the light intensity at the outer periphery in the Y direction lower than that at the inner periphery, the zero-order diffraction efficiency of the diffraction grating part 1222 of the correction hologram 1221 must be lower than the zero-order diffraction efficiency of the hologram lens 107. desirable.

  The dashed line in FIG. 13B is the 0th-order diffraction efficiency distribution of the correction hologram 1221 used when calculating the far-field image, and is slightly different from the example of FIG.

  FIG. 14 shows the result of calculating the focused spot profile on the information medium 51 at this time. In both (a) X direction and (b) Y direction, the height of the side lobe 381 is within about 1% of the maximum value of the main lobe 380. That is, it is possible to obtain an effect that the side lobe 381 of the light beam (condensed spot) on the information medium is further reduced, and a reproduced signal having excellent characteristics can be obtained.

  In the configuration including the collimating lens 122 as shown in FIG. 4, the number of components is reduced by forming the correction hologram 1221 on the peripheral portion of the surface of the collimating lens 122 as shown in FIG. An optical head device can be configured.

  In the above embodiment, the correction hologram has been described as a relief type. However, as disclosed in JP-A-61-189504 and JP-A-63-241735, niobium acid is used. Even if a part of the lithium substrate is exchanged with a proton or a liquid crystal cell is used, a phase modulation type correction hologram can be similarly produced.

  Further, in the correction hologram 1221, the diffraction grating portion 1222 is provided to reduce the transmittance, so that the transmittance may be reduced by forming a metal film or a dielectric film instead of the diffraction grating portion. In this case, for example, as shown in FIG. 16, instead of providing a correction hologram, a correction filter 1225 shown in FIG. 17 is provided on the surface of the polarization beam splitter 42. This correction filter 1225 has a metal film or a dielectric film provided on both sides as a transmittance reducing portion 1226, and a transmitting portion 1227 provided in the center between them. The X axis and Z axis inserted in FIGS. 16 and 17 are common. The effective diameter 1224 of the light beam is made wider than the width of the transmitting portion 1227. By forming the correction filter 1225 directly on the surface of the beam splitter 42 as described above, the number of components can be reduced, the number of optical components can be reduced, and the loss of light amount due to reflection can be avoided. be able to. Further, it is desirable that the metal film is formed of stable chromium or the like.

  It should be noted that the same effect can be obtained by providing a transmittance reducing portion on the surface of the collimator lens 122 instead of the surface of the beam splitter. In order to prevent the reflected light from returning to the semiconductor laser and inducing return noise, the normal to the surface of the correction filter may be inclined with respect to the optical axis. In particular, when the correction filter 1225 is provided on the surface of the beam splitter 42 as shown in FIG. 16, the beam splitter 42 is inclined. Further, as in the above-described embodiment using the diffraction grating, the transmission portion 1227 of the correction filter 1225 is made thick as shown in FIG. 18 and the optical path length is lengthened, thereby eliminating the adverse effect of the astigmatic difference of the semiconductor laser. Can also be obtained.

  Japanese Patent Application Laid-Open No. 58-85944 and Japanese Patent Application Laid-Open No. 62-67737 disclose a correction hologram or a correction filter seemingly similar to that used in the present embodiment. In the former, in order to reduce crosstalk due to the inclination of the optical axis, a radiation attenuating element having, for example, a transparent central portion and an edge that absorbs or reflects light is installed in the optical path, and the radiation intensity at the edge is reduced. Is reduced. In the latter case, a light splitting unit is provided in the optical path to form two sub beams in addition to the main beam. Here, in order to reduce crosstalk due to side lobes, the light splitting unit reduces the light intensity distribution of the main beam in the peripheral portion. However, these methods reduce the light intensity around the normal light beam and reduce the side lobe of the light beam. On the other hand, the present invention solves a problem unique to a bifocal lens using a hologram lens 107 that diffracts a part of incident light. In this case, as shown in FIG. 2B, the light amount distribution is large especially at the outer peripheral portion in the y direction, and there is a possibility that information reading becomes unstable. Thus, the present invention uses a correction hologram (or a correction filter) to return to a normal flat light intensity distribution. As a result, the present invention has a remarkable effect that the effect of reducing the side lobe is greater than that of the above-described conventional technology and that a light beam suitable for different substrate thicknesses can be formed by one lens. is there.

  As shown in FIG. 19, in the optical head device, a part of the light collected on the information recording surface and reading the information spreads largely on the photodetector. For example, when reproducing the information medium 51 (when the thickness of the substrate is t2) with the optical head device of the present invention using the hologram lens 107, the light collected on the information recording surface and reading the information is A servo signal and an information signal are read using the light transmitted through the hologram lens 107. Here, the light that has been condensed on the information recording surface and has read the information and has been diffracted by the hologram lens 107 spreads greatly like the first-order diffracted light 430 shown in FIG. Therefore, a large photodetector 75c (preferably having a size of 1 mm or more) is provided around the photodetector 75 for detecting the servo signal, receives these lights, and outputs the light of the photodetector 75 and the light detection. The sum of the outputs of the detector 75c is used as an information signal. Thereby, it is possible to obtain an effect that the S / N can be further improved and the frequency characteristics can be improved.

  The objective lens used in the present embodiment is basically configured by a combination of the objective lens 4 that refracts light and the hologram lens 107 that diffracts a part of the light. Therefore, the hologram lens 107 and the objective lens 4 are integrated by connecting them by using packaging means or by directly forming a lattice pattern of the hologram lens on the objective lens 4 as shown in FIG. Is also good. By doing so, the optical axis deviation between the hologram lens and the objective lens can be reduced, the off-axis aberration of the + 1st order diffracted light of the hologram lens can be reduced, and further weight reduction and cost reduction can be achieved. There is also an effect that can be.

  Furthermore, in a case where the hologram lens is designed to cause an aberration when tilted with respect to the optical axis, as shown in FIG. 20, the lattice pattern of the hologram lens 107 has a large curvature (a small radius of curvature) of the objective lens 4. ) Surface, that is, on the opposite side of the information medium (optical disk), it is also possible to obtain the effect that the aberration with respect to the optical axis of the hologram lens can be suppressed.

  Note that the phase of the zero-order diffracted light (transmitted light) of the grating pattern 107a in FIG. 20 is the average value of the amount of phase modulation given by the grating pattern 107a. Therefore, the surface of the average height 1070 (shown by a dotted line) of the height of the unevenness of the lattice of the lattice pattern 107a and the surface 1071 having no lattice pattern are continuously connected, and the average surface 1070 of the lattice of the lattice pattern 107a is formed. And the surface of the surface 1071 having no lattice pattern are designed so that the light beam can be focused to the diffraction limit through the substrate thickness t2.

  Further, as the next embodiment, it is also possible to design the hologram lens 107 into a convex lens type, focus + 1st-order diffracted light on the substrate thickness t2, and condense 0th-order light on the substrate thickness t1. is there. At this time, the + 1st-order diffraction efficiency at the outer periphery is set to almost 100%, and the diffraction efficiency at the inner periphery is smaller than 100%. In the description of the above-described embodiment, the 0th-order diffraction efficiency of the hologram lens 107 is replaced with the + 1st-order diffraction efficiency, and the + 1st-order diffraction efficiency of the hologram lens 107 is replaced with the 0th-order diffraction efficiency. A similar effect can be obtained with a similar configuration. In this embodiment, there is an effect that chromatic aberration is reduced or no longer occurs with respect to the substrate thickness t2. Conversely, it is also possible to condense this diffracted light on the information medium, receive the reflected light from the information medium with a photodetector, and obtain a tracking error signal by the so-called three-beam method from the output. is there. In this case, contrary to the above, it is desirable to blaze the diffraction grating portion 1222 in a direction in which the diffraction light intensity diffracted in the direction approaching the optical axis increases.

  FIG. 21 schematically shows another embodiment of the optical head device. This optical head device differs from the optical head device shown in FIG. 4 in that a finite optical system is used and that the beam splitter 363 is a flat plate. This difference has the effect of reducing the number of parts and cost. The light beam 3 from the radiation light source 2 has its outer peripheral portion changed in intensity by the correction hologram 1221, is reflected 90 ° by the beam splitter 363, and has an objective lens (bifocal lens) having the hologram lens lattice pattern 107. 4) (see FIG. 20), the light is focused on the information media 5, 51. The reflected light from the information media 5 and 51 passes through the beam splitter 363 and enters the photodetector 71.

  It is desirable that the light amount and phase modulation element (such as a correction filter) can be easily manufactured. The correction filter 1225 needs to form two types of patterns, that is, a transmittance reduction unit 1226 and a step. However, in the manufacturing method exemplified below, the masking step for patterning needs to be performed only once, which is simple and easy. It can be manufactured at low cost. Hereinafter, the transmittance reducing unit 1226 will be described using a metal film such as chromium as an example, but it can be replaced with a dielectric film. FIG. 22 shows a method of manufacturing the correction filter 1225.

  First, as shown in (a), after cleaning the transparent substrate 12252, a resist film 12251 is coated on the surface. The substrate 12252 is the beam splitter itself when forming the correction filter 1225 on the surface of the beam splitter 42 as in the example shown in FIG.

  Next, as shown in (b), a photomask 12253 in which a mask material (typically chromium) 12255 for blocking light is formed on a part of the substrate 12254 is placed on the resist film 12251. The resist film 12251 is exposed through the photomask 12253 for patterning.

  Next, as shown in (c), the resist film 12251 is developed except for the photomask 12253. As a result, the resist film remains only in the portion corresponding to the transmission portion 1227 of the correction filter 1225.

  Next, as shown in (d), etching is performed using the remaining resist film 12251 as a mask. Thus, only the portion of the surface of the substrate 12252 where the resist film 12251 is not present is etched. Thus, a step having the resist film 12251 thereon is formed.

  Next, as shown in (e), a metal film 12261 is formed by vapor deposition or sputtering. The thickness of the metal film 12261 is smaller than the height of the step.

  Finally, as shown in (f), the resist film 12251 is removed with an organic solvent or the like. At this time, the metal film formed on the resist film 12251 can be simultaneously removed (lift-off). Thus, the correction filter 1225 is completed.

  FIG. 23 shows another method of manufacturing the correction filter 1225. First, as shown in (a), after cleaning the transparent substrate 12252, a metal film 12261 is deposited on the surface of the substrate 12251.

  Next, as shown in (b), a resist film 12251 is further applied on the metal film 12261.

  Next, as shown in (c), a photomask 12253 in which a mask material (generally chrome) 12255 for shielding light is formed on a part of the substrate 12254 is placed on the resist film 12251. The mask material 12255 is formed at portions other than the central portion. The resist film 12251 is exposed through the photomask 12253 for patterning.

Next, the photomask 12253 is removed, and the resist film 12251 is developed. Thus, as shown in (d), the resist film 12251 remains only in the unexposed portions.
Next, the metal film 12261 is etched using the remaining resist film 12251 as a mask. Thus, as shown in (e), the portion of the metal film 12261 corresponding to the transmission portion 1227 is removed.

Next, as shown in (f), a transparent film 12262 such as SiO ₂ is formed. The thickness of the film 12262 is larger than the thickness of the metal film 12261.

Next, as shown in (g), the resist film 12251 is removed by dissolving with a solvent or the like. At this time, the SiO ₂ on the resist film 12251 is simultaneously removed (lift-off). Thus, the correction filter 1225 is completed.

  The objective lens design method, optical head device, and optical disk device according to the present invention are used for recording / reproducing or erasing information stored on an optical medium or a magneto-optical medium (information medium).

FIG. 4 is a perspective view for explaining a state of spread of light emitted from a semiconductor laser. It is a graph of the light quantity distribution of a light beam, (a) shows the light quantity distribution of an X direction, (b) shows the light quantity distribution of a Y direction. It is a graph of the light quantity distribution of the condensing spot on an information medium, (a) shows the light quantity distribution in the X direction, (b) shows the light quantity distribution in the Y direction. FIG. 1 is a schematic sectional view of an optical head device according to an embodiment of the present invention. It is a top view showing the hologram pattern of a hologram lens. It is sectional drawing of a hologram lens. 5 is a graph of a light amount distribution of a converging spot on an information medium. It is sectional drawing of the compound objective lens for demonstrating the condensing state of the transmitted light at two focuses, (a) is a case where transmitted light transmits other than the grating pattern of a hologram lens, (b) Indicates a case where the transmitted light is transmitted only within the grating pattern of the hologram lens. It is a figure for demonstrating the effect | action of a correction | amendment hologram, (a) shows the outline explaining light intensity suppression in an outer peripheral part by a correction | amendment hologram, (b) shows a correction | amendment hologram. FIG. 3 is a schematic sectional view of an optical head device including a correction hologram. It is a schematic plan view of a correction hologram. It is a graph which shows the transmittance distribution of a Y direction. It is a graph of the light amount distribution of the light beam when it has a correction hologram, (a) shows the light amount distribution in the X direction, and (b) shows the light amount distribution in the Y direction. It is a graph of the light quantity distribution of the condensed spot on the information medium, (a) shows the light quantity distribution in the X direction, and (b) shows the light quantity distribution in the Y direction. FIG. 3 is a cross-sectional view of a collimator lens provided with a correction hologram. It is sectional drawing of an optical head device. It is a top view of a correction filter. FIG. 3 is a cross-sectional view of a correction filter. FIG. 3 is a perspective view for explaining a state of light on a photodetector. It is a schematic sectional drawing of a compound objective lens. FIG. 11 is a cross-sectional view of another embodiment of the optical head device including the compound objective lens. It is a figure showing the manufacturing method of a correction filter. It is a figure showing another manufacturing method of a correction filter.

Explanation of reference numerals

  2 radiation light source, 3 light beam, 4 objective lens, 5, 51 information medium, 71 photodetector, 36 beam splitter, 107 hologram lens, 121 convergent lens, 122 collimating lens, 1221 correction hologram, 1225 correction filter.

Claims (9)

A radiation source,
An objective lens for condensing light emitted from the radiation light source on an optical disc through different substrate thicknesses,
Comprising a photodetector for receiving light reflected back from the optical disc,
An optical head device, comprising: reproducing information on an optical disk having a different substrate thickness by receiving light reflected from the optical disk and returned by the optical detector,
The objective lens divides the objective lens surface into at least a first and a second plurality of regions according to a distance from a light axis of the incident light to a passing region of the incident light, and the first region is formed on an outer peripheral portion. A first hologram is formed in the first area, and a first-order diffracted light obtained by diffracting light passing through the first area by the first hologram is applied to an optical disk having a transparent substrate thickness t2. By providing a second hologram composed of concavo-convex annular zones in the second area, the light passing through the second area is transmitted to an optical disk having a transparent substrate thickness of either t1 or t2. T2 <t1, the orbicular zone has a concentric shape centered on the optical axis of the objective lens, and the orbicular zone has a larger curvature of the objective lens, that is, a radius of curvature. Formed on the smaller side of An optical head apparatus characterized by suppressing aberration that occurs when tilted with respect to the optical axis by. 2. The optical head device according to claim 1, wherein both the optical disk corresponding to the transparent substrate thickness t1 and the optical disk corresponding to the transparent substrate thickness t2 different from t1 are recorded and reproduced. A radiation light source, an objective lens that receives a light beam emitted from the radiation light source, and converges on an optical disk into a minute spot, and a light that receives an optical beam reflected and diffracted by the optical disk and outputs an electric signal according to the light amount Detector, provided in the optical path from the radiation light source to the objective lens, comprising a hologram partially having a diffraction grating portion,
The objective lens divides the objective lens surface into at least a first and a second plurality of regions according to a distance from a light axis of the incident light to a passing region of the incident light, and the first region is formed on an outer peripheral portion. A first hologram is formed in the first area, and a first-order diffracted light obtained by diffracting light passing through the first area by the first hologram is applied to an optical disk having a transparent substrate thickness t2. By providing a second hologram composed of concavo-convex annular zones in the second area, the light passing through the second area is transmitted to an optical disk having a transparent substrate thickness of either t1 or t2. T2 <t1, the orbicular zone has a concentric shape centered on the optical axis of the objective lens, and the orbicular zone has a larger curvature of the objective lens, that is, a radius of curvature. Formed on the smaller side of Suppressing aberration generated when the inclined with respect to the optical axis by,
The hologram is a transmission portion without the diffraction grating portion near the optical axis connecting the radiation light source and the center of the objective lens, the diffraction grating portion is formed in a region away from the optical axis,
Of the diffracted light diffracted from the diffraction grating portion, the diffracted light diffracted in the direction approaching the optical axis is focused on the optical disk by the objective lens, and the light reflected from the optical disk and returned is received by the photodetector. An optical head device for detecting a tracking error signal. The light beam emitted from the radiation light source is perpendicular to the radiation direction and has a different rate of change in light intensity in two directions perpendicular to each other, and the hologram has a direction in which the rate of change in light intensity is the gentlest. When a direction perpendicular to both the Y direction and the radiation direction is defined as an X direction, a region forming the diffraction grating portion is provided in a direction away from the optical axis in the Y direction. The optical head device according to claim 3, wherein The optical head device according to claim 3, wherein the diffraction grating portion is blazed in a direction in which the intensity of the first-order diffracted light diffracted in a direction approaching the optical axis increases. Further, a collimating lens for collimating a light beam emitted from the radiation light source is provided between an optical path from the radiation light source to the objective lens, and a light intensity correction unit is integrally formed on a surface of the collimating lens. The optical head device according to claim 3, 4 or 5, wherein: In the objective lens, a first region of the objective lens designed to converge light only to an optical disk having a specific transparent substrate thickness t2 is an outermost peripheral region farthest from the optical axis. 7. The optical head device according to claim 1, wherein the numerical aperture NA1 for the transparent substrate thickness t1 is made smaller (NA1 <NA2) than the numerical aperture NA2 for the transparent substrate thickness t2. . 8. The optical head device according to claim 1, wherein the + 1st-order diffraction efficiency of the first hologram is set to approximately 100%. Using the optical head device according to any one of claims 1 to 8,
Focus the light emitted from the radiation source on the optical disc,
A tracking error signal is detected by receiving light reflected from the optical disk and returned by a photodetector,
An optical disk device for reproducing information from an optical disk.

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