JP4201196B2 - Optical head device and optical disk device - Google Patents

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 optical disks with pit-like patterns as high-density and large-capacity storage media has been put into practical use while expanding applications with digital audio disks, video disks, document file disks, and data files. . The mechanism by which information is successfully recorded and reproduced on an optical disc through a finely focused light beam 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, can be broadly divided into light-collecting properties that form diffraction-limited microspots, focus control and tracking control of the optical system, and detection of pit signals. The These are manifested by combinations of various optical systems and photoelectric conversion detection methods depending on the purpose and application.

  2. Description of the Related Art In recent years, optical discs having higher storage capacity than conventional ones have been developed due to advances in optical system design technology and shorter wavelengths of semiconductor lasers as light sources. As an approach for increasing the density, it has been studied to increase the numerical aperture (NA) on the optical disc side of the condensing optical system that finely focuses the light beam onto the optical disc. At that time, the problem is an increase in the amount of aberration caused by the tilt of the optical axis (so-called tilt). Increasing the numerical aperture NA increases the amount of aberration that occurs with respect to tilt. In order to prevent this, the thickness (base material thickness) of the substrate of the optical disk may be reduced. For example, in order 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 disc. For this reason, it is considered that the thickness of the substrate of the next generation high-density optical disc is thinner than many conventional optical discs including a commercially available compact disc (CD). Naturally, an optical disc apparatus capable of recording and reproducing both a conventional optical disc and a next-generation high-density optical disc is required. For this purpose, an optical head device including a condensing optical system capable of condensing a light beam up to the diffraction limit on an optical disk having a different substrate thickness is required.

The inventor has 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 the optical head, the light beam from the light source is converted into parallel light, and condensed onto a minute spot by the objective lens and irradiated onto the 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 an optical head for a plurality of optical discs having different substrate thicknesses, optical discs having different thicknesses are used by using a bifocal lens in which an objective lens and a hologram lens that diffracts a part of incident light are combined. A condensing spot that is condensed to the diffraction limit is formed on each of them. Since the hologram lens diffracts a part of incident light, for example, it has a concentric lattice pattern, and transmitted light (0th order diffracted light) has sufficient intensity. The light diffracted by the hologram lens and the light not diffracted are condensed 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 action, the positions of the two focal points in the optical axis direction are different, and when information is recorded / reproduced at one focal point, the light beam having the other focal point as the focal point is The light intensity is wide and the light intensity is small, and recording / reproduction is not affected.
JP-A-7-98431 JP 61-189504 A JP-A-63-241735 JP 58-85944 A JP-A-62-67737

The above-described optical head device using the 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. In particular, in order to reduce the size of the optical head device, it is desirable to use a semiconductor laser as the light source, but there is a problem due to the use of the semiconductor laser. As shown in FIG. 1, the semiconductor laser emits a light beam from a light emitting point 2002 near the end face of the active layer 2001. Here, in the far-field image of the light beam, the spread angle θ _{Y in} the Y direction orthogonal to the active layer 2001 is wider than the spread angle θ _{X in the} X direction parallel to the active layer 2001. FIG. 2 shows the light intensity distribution 2003 in the X direction (a) and Y direction (b) of the light beam emitted from the semiconductor laser (when the light beam diameter is φ4 mm). As described above, the distribution is greatly different in both directions. When this light beam is incident on the above-described hologram lens that diffracts a part of the incident light, the light intensity at the outer peripheral portion is higher in the X direction (a) and Y direction (b) as shown by the hatched portions, respectively. Stronger than the circumference. Since the spread 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 light intensity distribution in the X direction (a) and the Y direction (b) of the focused spot on an information medium with a thin substrate thickness when a hologram lens that diffracts a part of incident light is used. The calculation results are shown. Here, the light intensity is normalized with the maximum value of the main lobe 380 as 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 the recording pit shape and the reproduction signal. The light intensity of the side lobe 381 when normalized with the maximum value of the light intensity of the main lobe 380 being 100 is sufficiently low, about 1% in the X direction, but about 4% in the Y direction. Even if the height of the side lobe is about 4%, it is possible to read the information signal sufficiently, but in order to read the information signal more stably against disturbances such as vibration and temperature change, the side lobe is more It is better to mitigate the effect by lowering it.

The first object of the present invention is to condense a light beam to the diffraction limit on information media (optical discs) having different substrate thicknesses by using a hologram lens with sufficient intensity of transmitted light. An object of the present invention is to provide an optical head device that can alleviate the influence of the increase in the light intensity at the outer peripheral portion and obtain a more stable information signal.
The second object of the present invention is an improvement in which a light beam can be condensed to the diffraction limit on information media (optical disks) having different substrate thicknesses by using a hologram lens having sufficient intensity of transmitted light. An optical head device is provided.

An optical head device according to the present invention is a light for recording or reproducing information on a first information medium having a first thickness substrate and a second information medium having a second thickness substrate. A head device,
A light source, an objective lens that receives light emitted from the light source and focuses light on an information recording surface of the first information medium or the second information medium; and the first information medium or the A photodetector that receives the light reflected by the second information medium and converts the light into an electrical signal. The objective lens is on the information recording surface of the information medium having a substrate, a objective lens for collecting the light through the substrate, comprising a refraction lens, and a hologram lens having an aberration correction action , the hologram lens comprises a plurality of regions, said plurality of regions, a first region, including a second region closer to the optical axis of the objective lens than that of the first region. The hologram lens is integrally formed on the surface having the larger curvature of the refractive lens, that is, the surface having the smaller radius of curvature, and both the light that has passed through the first region and the light that has passed through the second region. Is condensed on the information recording surface of the first information medium having the first thickness substrate via the first thickness substrate, and only the light that has passed through the second region is The light is condensed on the information recording surface of the second information medium having the substrate having the thickness of 2 through the substrate having the second thickness . The first thickness is smaller than the second thickness, and the numerical aperture for condensing light through the first thickness substrate is the second thickness substrate. have magnitude than the numerical aperture for the light is condensed through. The light source is a semiconductor laser having an astigmatic difference, and includes a light intensity correction element in an optical path from the light source to the objective lens, and the light intensity correction element includes a liquid crystal cell, and the liquid crystal cell Reduces aberrations .

In the optical head device, it is preferable that the hologram lens has a concentric relief shape.

  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 is increased.

In the optical head, preferably, the light emitted from the light source is passed through the substrate having the first thickness on the information recording surface of the first information medium having the substrate having the first thickness. And when the light is condensed through the second thickness substrate on the information recording surface of the second information medium having the second thickness substrate. In this case, light is received by a common photodetector and converted into an electrical signal.

An optical disc apparatus according to the present invention records or reproduces information on the first information medium having the first thickness substrate and the second information medium having the second thickness substrate. using the optical head device for performing the light emitted from the light source is condensed onto the information recording surface of the first information medium or the second information medium, the first information medium or the second the light reflected back from the second information medium to detect a tracking error signal by receiving at the photodetector, reproducing information of the first information medium or the second information medium.

  An optical disc apparatus according to the present invention uses any one of the above-described optical head devices 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 reproduction on the information medium is performed.

In the present invention, the following effects can be obtained.
(1) By combining a hologram lens that diffracts a part of incident light and an objective lens, the light is 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. By using this bifocal lens, an optical head device with a small number of parts and a small size, a light weight and a low cost can be used. This can be done with 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 (condensing spot) is further reduced on the information medium, and a reproduction signal having excellent characteristics can be obtained. it can.
(3) When the semiconductor laser is used as the light source by thickening the portion of the light intensity correction means such as the correction hologram or the correction filter which has a high zero-order diffraction efficiency or high transmittance and lengthening the optical path length, The adverse effect of point difference can be removed or reduced.
(4) In combination with a light intensity correction means such as a correction hologram or a correction filter, a large (preferably φ1 mm or more) light detector is provided around the light detector for servo signal detection to receive the light from the outer periphery, By using the sum of both photodetectors as an information signal, the S / N ratio can be further improved and the frequency characteristics can be improved.
(5) Light intensity correction means such as a correction hologram or a correction filter can be formed on the surface of the objective lens or collimating lens to reduce the number of parts and further reduce the price.
(6) When the hologram lens is formed on the objective lens surface, the average surface of the lattice of the lattice pattern and the surface of the surface without the lattice pattern are continuously connected, and the average surface of the lattice of the lattice pattern and the lattice pattern Good condensing characteristics can be obtained by designing the surface with no surface so that the light beam can be condensed almost to the diffraction limit.

Hereinafter, a correction hologram or a correction filter and an optical system for condensing a light beam at two different positions will be described first with reference to the accompanying drawings.
FIG. 4 shows an optical head device according to the first embodiment of the present invention. This optical head device is characterized by forming a bifocal lens from the objective lens 4 and the hologram lens 107, and providing a correction hologram 1221 for reducing the light quantity at the outer periphery. In this optical head device, the light beam 3 emitted from the radiation light source 2 such as a semiconductor laser becomes substantially parallel light by the collimating lens 122, passes through the correction hologram 1221, and the light intensity at the outer peripheral portion decreases. The light beam further passes through the polarization beam splitter 42 and is circularly polarized by the quarter wavelength plate 15. Next, the light beam enters the hologram lens 107 and the objective lens 4 and is condensed on the thick information medium 5 or the thin information medium 51 positioned 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 “substrate thickness” refers to the thickness from the surface on which the light beam is incident on the information medium to the information recording surface. The term “condensing” is used here, but “condensing” is defined as “converging diverging light or parallel light to a diffraction-limited microspot”.

  As schematically shown in FIGS. 5 and 6, the hologram lens 107 is formed on a substrate 9 that is transparent to the light beam 3, and has a ring-shaped lattice pattern 107a in the central portion and a lattice pattern around it. The region 107b has no area. The grating 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 sufficient intensity. For example, when the hologram lens 107 is formed in a concavo-convex shape as shown in FIG. 4, the height h of the (relief type) concavo-convex is smaller as h <λ / (n−1), that is, This can be easily realized by making the amplitude of the phase change given to the light beam by the grating 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. In this way, by allowing the transmitted light to have 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 kept low. It has an effect that can be.

  The hologram lens 107 includes a grating pattern 107a and a region 107b having no grating pattern. The phase of the 0th-order diffracted light (transmitted light) of the grating pattern 107a is an average value of the phase modulation amount given by the grating pattern 107a. On the other hand, it is desirable to improve the light condensing performance by matching the phase of the transmitted light of the region 107b having no lattice pattern to the same extent. Therefore, for example, when the grating pattern 107a of the hologram lens 107 is a relief type, as shown in FIG. 6, the height of the surface of the area 107b without the grating pattern is adjusted to the level of the average of the irregularities of the grating pattern portion.

  The light beam reflected by the information medium 5 or 51 follows the original optical path in reverse. The transmitted light 61 is again transmitted through the hologram lens 107 as shown by the solid line, and the + 1st order diffracted light 64 is again diffracted as the + 1st order diffracted light by the hologram lens 107 as shown by the broken line. The light passes through the same optical path as after passing through 42 and is reflected by the polarization beam splitter 42. The reflected light is collected by the converging lens 121, and after the wavefront is converted by a wavefront conversion means such as a cylindrical lens 131 so that a servo signal such as a focus error signal and a tracking error signal can be obtained, light detection is performed. Incident on the vessel 71. A servo signal (focus error signal and tracking error signal) and an information signal can be obtained by calculating the output of the photodetector 71.

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

  As shown in FIG. 8A, the objective lens 4 has a numerical aperture NA of 0.6 or more and a thickness t2 of the substrate 37 when the light beam 61 transmitted through the hologram lens 107 without being diffracted is incident. It is designed so that a diffraction limited condensing spot 38 a can be formed on a thin optical disk 51. In this case, the grating pattern 107 a of the hologram lens 107 is formed only within a diameter smaller than the opening determined by the objective lens 4. Therefore, no diffraction occurs at the portion 107b where the grating pattern of the hologram lens 107 is not formed, and the light amount of the high NA condensing spot 38a increases.

  On the other hand, FIG. 8B shows that the focused spot 38b can be focused at the diffraction limit on the information medium 5 having a low NA and a thick substrate (thickness t1). The + 1st order diffracted light 64 diffracted by the hologram lens 107 is condensed on the information medium 5 by the objective lens 4. Here, the + 1st-order diffracted light 64 is subjected to aberration correction so as to be narrowed down to the diffraction limit through the substrate 37 having a thickness t1. In the design method of the hologram lens 107 having such an aberration correcting action, for example, a spherical wave that diverges from the condensing spot 38b passes through the substrate 37 having a thickness t1 and then passes through the objective lens 4 to pass through the hologram lens 107. It is only necessary to calculate an interference pattern (hologram lens grating pattern 107a) between the light beam transmitted through the formed transparent substrate 9 and the light beam obtained by inverting the phase of the light beam 3 in FIG. Then, for example, the hologram lens 107 can be easily manufactured by a computer generated hologram (CGH) method or the like. In this way, by combining the hologram lens 107 that diffracts a part of incident light and the objective lens 4, a condensing spot that is condensed to the diffraction limit on optical disks having different substrate thicknesses (t 1 and t 2). A bifocal lens capable of forming the lens 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, the light having the other focal point as a condensing point. The beam is greatly expanded, the light intensity is small, and recording / reproduction is not affected. For example, as shown in FIG. 8A, when the condensing spot 38a is at the in-focus position with respect to the information medium 51, the + 1st order diffracted light 64 is greatly spread on the information recording surface of the information medium 51, and the recording is performed. Does not affect playback. The same applies to the case of FIG. 8B.

  The difference between the two focal positions affects the recording / reproduction when the information beam is recorded / reproduced at one focal spot, the light beam having the other focal point as the condensing point is greatly expanded and the light intensity is small. In order to avoid such a situation, it is desirable to make it as large as possible with 50 μm or more. Further, since the substrate thickness t1 of a compact disc (CD), laser disc (LD), etc. is about 1.2 mm and the substrate thickness t2 of the high-density optical disc is considered to be appropriate from 0.4 mm to 0.8 mm, the objective lens In consideration of the movable range of the actuator responsible for the focus servo operation, it is desirable that the difference between the two focal positions does not greatly exceed the difference of about 0.8 mm between t1 and t2. Therefore, as shown in FIG. 8A, when the focal length of the focused spot 38a corresponding to the thin substrate with high NA is shortened, 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, the objective lens 4 has a diffraction limit on an optical disk having a thin substrate thickness (t2) when a light beam 61 transmitted through the hologram lens 107 without being diffracted is incident. Designed to form a focused spot. Further, the grating pattern 107 a of the hologram lens 107 of the present embodiment is formed only within a diameter smaller than the opening determined by the objective lens 4. Accordingly, no diffraction occurs at the portion of the hologram lens 107 where the grating pattern 107b is not formed. In this way, by combining the hologram lens 107 that diffracts a part of incident light and the objective lens 4, a condensing spot that is condensed to the diffraction limit on optical disks having different substrate thicknesses (t 1 and t 2). The bifocal lens which can form is realizable.

  The transmitted light 61 and the + 1st order diffracted light passing through the hologram lens 107 reflected by the information media 5 and 51 again pass through the same optical path as when passing through the polarizing beam splitter 42 first. And then collected by the converging lens 121. A servo signal is detected by the photodetector 71 using the condensed light beam. Therefore, the condensing point 39 on the light detector side of the light beam reflected from the two focal points is coincident with the point of emission of the radiation light source 2 in a conjugate relationship. For this reason, a single servo signal detection means and photodetector 71 can be used in common, and an optical head device with a small number of parts and a compact, lightweight, low-cost optical head device with different substrate thicknesses can be used. Recording / reproducing can be performed with one optical head device.

  As a detection method of the focus error signal, an arbitrary method such as a spot size detection method (SSD method: JP-A-2-185722), an astigmatism method, or 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 used.

  One of the features of the present invention is that an element such as a correction hologram 1221 or a 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 the optical system to change the light quantity and phase as desired, thereby reducing the light intensity at the outer periphery. .

  FIG. 9 shows an optical path in the forward path from the radiation source 2 to the objective lens 4. For example, as shown in (b), the correction hologram 1221 is provided with a transmission part 1223 that transmits all light near the optical axis 3000 and a diffraction grating part 1222 away from the optical axis 3000. . Then, as shown in (a), in the forward path, all the light near the optical axis of the light beam 3 emitted from the radiation light source 2 is transmitted, and part of the light away from the optical axis is diffracted and transmitted. Reduce the rate.

  Here, the phase of the 0th-order diffracted light (transmitted light) of the diffraction grating unit 1222 is an average value of the phase modulation amounts given by the diffraction grating unit 1221. On the other hand, it is desirable to improve the light condensing performance by the objective lens 4 by matching the phase of the transmitted light in the region 1223 without the lattice pattern to the same extent. Therefore, for example, as shown in FIG. 9B, when the grating pattern of the hologram is a relief type, the transmission portion 1223 has a level about the average of the unevenness of the diffraction grating portion 1222 (the average of the top and bottom). Match the height of the surface.

  However, when a semiconductor laser is used for the radiation 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 provided. Is made higher than the average of the diffraction grating portion 1222. In other words, the thickness of the transmission part 1223a is made thicker than that of the diffraction grating part 1222, and the optical path length is made longer. As a result, the adverse effect of the astigmatic difference of the semiconductor laser can be reduced, and the light collection 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 with a narrow divergence angle of the emitted light enters the active layer of the semiconductor laser. Therefore, if the converging positions in both directions are made to coincide with each other by increasing the convex lens action of the optical system in the direction perpendicular to this, that is, in the direction in which the spread angle of the emitted light is wide, the wavefront aberration can be reduced. As will be described later, since it is desirable to match the Y direction in FIG. 10 with a wide divergence angle, it is desirable to have a convex lens action in this Y direction. This is the reason why the transmissive portion 1223a should be thick.

  Further, the diffracted light 1224 diffracted from the correction hologram 1221 becomes unnecessary stray light. Therefore, it is desirable not to enter the opening of the objective lens 4 or to design so that it 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 aperture of the objective lens 4, the grating pitch of the diffraction grating portion 1222 is set to 5 μm or less, preferably 2 μm or less, or as shown in FIG. After the diffraction grating portion 1222 is blazed in a direction in which the intensity of 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 collect this diffracted light on an information medium, receive reflected light from the information medium with a photodetector, and obtain a tracking error signal by the so-called three-beam method from its output. . In this case, it is desirable to blaze the diffraction grating part 1222 in a direction in which the intensity of diffracted light that diffracts in the direction approaching the optical axis increases, contrary to the above case.

  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. A circle representing the optical axis 3000 and a circle representing the light beam effective diameter 1224 are virtually drawn for the sake of explanation, and are not actually formed in the correction hologram 1221. The coordinate axes are the same as those in FIGS. In order to reduce the amount of light in the direction away from the optical axis in the direction of the widening angle of the light beam, that is, in the Y direction in FIG. 1, the portion away from the optical axis in the Y direction is also the diffraction grating 1222 in FIG. Is desirable.

  Further, since the contour line of the light intensity of the far field image (FFP) is elliptical, the amount of light is effectively increased by making the boundary line between the diffraction grating portion 1222 and the transmission portion 1223 convex outward from the optical axis 3000. The effect that it can be utilized for can be acquired.

  FIG. 12 shows an example of a change in the Y-axis direction of the zero-order diffraction efficiency (transmittance) of the correction hologram 1221. The origin is the intersection of the optical axis 3000 and the correction hologram 1221. The 0th-order diffraction efficiency is lowered toward the outer periphery. Thus, 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. 13.

  FIG. 13 shows the light intensity distribution in the X direction (a) and the Y direction (b). By reducing the light intensity in the Y direction with the correction hologram 1221, the light intensity distribution (b) in the Y direction is different from that in FIG. 2. That is, the light intensity at the outer peripheral portion in the Y direction is lower than that at the inner peripheral portion.

  In order to make the light intensity at the outer peripheral portion in the Y direction lower than that at the inner peripheral portion, the 0th-order diffraction efficiency of the diffraction grating portion 1222 of the correction hologram 1221 should be lower than the 0th-order diffraction efficiency of the hologram lens 107. desirable.

  The broken 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 in FIG.

  FIG. 14 shows the result of calculating the condensing spot profile on the information medium 51 at this time. The height of the side lobe 381 is within about 1% of the maximum value of the main lobe 380 in both the (a) X direction and (b) Y direction. That is, the side lobe 381 of the light beam (condensing spot) is further lowered on the information medium, and an effect that a reproduction signal having excellent characteristics can be obtained can be obtained.

  In the configuration including the collimating lens 122 as shown in FIG. 4, the number of parts is reduced by forming the correction hologram 1221 around 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 Japanese Patent Application Laid-Open Nos. 61-189504 and 63-241735, niobic acid is used. A phase-modulated correction hologram can be similarly produced by exchanging a part of the lithium substrate or using a liquid crystal cell.

  Further, in the correction hologram 1221, the diffraction grating portion 1222 is provided to reduce the transmittance. Therefore, 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, a correction filter 1225 shown in FIG. 17 is provided on the surface of the polarization beam splitter 42 instead of providing a correction hologram. The correction filter 1225 is provided with a metal film or a dielectric film on both sides as a transmittance reduction unit 1226, and a transmission unit 1227 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 transmission part 1227. By forming the correction filter 1225 directly on the surface of the beam splitter 42 in this way, the number of parts can be reduced, the number of optical parts can be reduced, and the loss of light quantity due to reflection can be avoided. be able to. The metal film is preferably 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 line of the correction filter surface 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 tilted. Further, similarly to the embodiment using the above-described diffraction grating, the transmissive portion 1227 of the correction filter 1225 is thickened as shown in FIG. 18 and the optical path length is lengthened, thereby removing the adverse effect of the astigmatic difference of the semiconductor laser. The effect that it is possible can also be acquired.

  Japanese Patent Application Laid-Open No. 58-85944 and Japanese Patent Application Laid-Open No. 62-67737 disclose apparently similar correction holograms or correction filters used in this embodiment. In the former, in order to reduce crosstalk due to the tilt of the optical axis, for example, a radiation attenuation element consisting of a transparent central part and an edge part that absorbs or reflects light is installed in the optical path, and the radiation intensity at the edge part Reduce. In the latter case, in order to form two sub beams in addition to the main beam, the light dividing means is installed in the optical path. Here, in order to reduce crosstalk due to side lobes, the light splitting means reduces the light intensity distribution of the main beam at the peripheral portion. However, both of these 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 peculiar 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 quantity distribution is particularly large at the outer periphery in the y direction, and information reading may become unstable. Therefore, the present invention restores a normal flat light intensity distribution using a correction hologram (or correction filter). As a result, the present invention has a greater effect of reducing side lobes than the above-described prior art, and can obtain a remarkable effect that a light beam suitable for different substrate thicknesses can be formed with one lens. is there.

  As shown in FIG. 19, in the present optical head device, a part of the light that has been collected on the information recording surface and read the information largely spreads on the photodetector. For example, in the optical head device of the present invention using the hologram lens 107, when reproducing the information medium 51 (when the thickness of the substrate is t2), the light collected on the information recording surface and reading the information is Servo signals and information signals are read using the light transmitted through the hologram lens 107. Here, the light that is collected on the information recording surface and read the information is diffracted by the hologram lens 107 and spreads widely as the first-order diffracted light 430 shown in FIG. Therefore, a large (preferably φ1 mm or more) photodetector 75c is provided around the photodetector 75 for detecting the servo signal to receive these lights, and the output of the photodetector 75 and the optical detection are detected. The sum of the outputs of the device 75c is used as an information signal. As a result, it is possible to further improve the S / N ratio and improve the frequency characteristics.

Next, among the embodiments of the present invention, an embodiment in which a hologram lens and a refractive lens are integrally formed will be described with reference to the accompanying drawings.
The objective lens used in the present embodiment is basically composed of a combination of the objective lens 4 that refracts light and the hologram lens 107 that diffracts part of the light. Therefore, the hologram lens 107 and the objective lens 4, as shown in FIG. 20, on the objective lens 4 may be integrated by Rukoto pollock directly work the grating pattern of the hologram lens. By doing so, the optical axis shift 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 further reduced, and further weight reduction and cost reduction can be achieved. There is also an effect that can be.

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

  Note that the phase of the 0th-order diffracted light (transmitted light) of the grating pattern 107a in FIG. 20 is an average value of the phase modulation amount given by the grating pattern 107a. Therefore, the average surface 1070 (indicated by dotted lines) of the height of the unevenness of the lattice of the lattice pattern 107a and the surface 1071 without the lattice pattern are continuously connected, and the average surface 1070 of the lattice of the lattice pattern 107a. And the surface 1071 having no grating pattern are designed so that the light beam can be condensed almost to the diffraction limit through the substrate thickness t2.

  Furthermore, as a next embodiment, the hologram lens 107 can be designed as a convex lens type, and + 1st order diffracted light can be condensed with respect to the substrate thickness t2, and 0th order light can be condensed with respect to the substrate thickness t1. is there. At this time, the + 1st order diffraction efficiency of the outer peripheral portion is set to approximately 100%, and the diffraction efficiency of the inner peripheral portion is set to be smaller than 100%. In the description of the above-described embodiment, the correction hologram 1221 is obtained by replacing the 0th-order diffraction efficiency of the hologram lens 107 with the + 1st-order diffraction efficiency and replacing the + 1st-order diffraction efficiency of the hologram lens 107 with the 0th-order diffraction efficiency. The same effect can be obtained with the same 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 collect the diffracted light on an 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 that the diffraction grating portion 1222 be blazed in a direction in which the intensity of diffracted light that diffracts in the direction approaching the optical axis increases.

  FIG. 21 shows an outline of another embodiment of the optical head device. The optical head device differs from the optical head device shown in FIG. 4 in that a finite optical system is used and the beam splitter 363 is a flat plate. This difference has the effect of reducing the number of parts and reducing costs. The light beam 3 from the radiation light source 2 is changed in light intensity at the outer peripheral portion by the correction hologram 1221, reflected by 90 ° by the beam splitter 363, and an objective lens (bifocal lens) provided with a grating pattern 107 of the hologram lens. ) 4 (see FIG. 20), the light is condensed on the information media 5 and 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 a light quantity 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 step with the transmittance reduction unit 1226. However, in the manufacturing method exemplified below, the masking process for patterning needs to be performed only once. It can be manufactured at low cost. Hereinafter, the transmittance reduction unit 1226 will be described using a metal film of chromium or the like as an example, but may be replaced with a dielectric film. FIG. 22 shows a method for manufacturing the correction filter 1225.

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

  Next, as shown in (b), a photomask 12253 on which a mask material (generally 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 for patterning through the photomask 12253.

  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. As a result, only the portion without the resist film 12251 on the surface of the substrate 12252 is etched. Thus, a step with the resist film 12251 placed 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 removed at the same time (lift-off). Thus, the correction filter 1225 is completed.

  FIG. 23 shows another method for manufacturing the correction filter 1225. First, as shown in (a), after the transparent substrate 12252 is washed, 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 on which a mask material (generally chromium) 12255 that blocks light is formed on a part of the substrate 12254 is placed on the resist film 12251. Mask material 12255 is formed in a portion other than the central portion. The resist film 12251 is exposed for patterning through the photomask 12253.

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 portion that is not exposed.
Next, the metal film 12261 is etched using the remaining resist film 12251 as a mask. Thus, as shown in (e), the metal film 12261 corresponding to the transmission part 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 it with a solvent. At this time, the SiO ₂ on the resist film 12251 is removed at the same time (lift-off). Thus, the correction filter 1225 is completed.

  The objective lens designing method, the optical head device, and the 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).

It is a perspective view for demonstrating the mode of the spreading | diffusion of the emitted light of a semiconductor laser. It is a graph of the light quantity distribution of a light beam, (a) shows the light quantity distribution of X direction, (b) shows the light quantity distribution of 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 of X direction, (b) shows the light quantity distribution of Y direction. It is a schematic sectional drawing of the optical head apparatus of embodiment of this invention. It is a top view showing the hologram pattern of a hologram lens. It is sectional drawing of a hologram lens. It is a graph of the light quantity distribution of the condensing spot on an information medium. It is sectional drawing of the compound objective lens for demonstrating the condensing condition of the transmitted light in two focus, (a) is a case where transmitted light permeate | transmits besides the grating | lattice pattern of a hologram lens, (b) Indicates a case where 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 hologram, (a) shows the outline explaining light intensity suppression in an outer peripheral part by a correction hologram, (b) shows a correction hologram. It is a schematic sectional view of an optical head device provided with a correction hologram. It is a schematic plan view of a correction hologram. It is a graph which shows the transmittance | permeability distribution of a Y direction. It is a graph of the light quantity distribution of a light beam when a correction hologram is provided, (a) shows the light quantity distribution in the X direction, and (b) shows the light quantity distribution in the 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 of X direction, (b) shows the light quantity distribution of Y direction. It is sectional drawing of the collimating lens provided with the correction hologram. It is sectional drawing of an optical head apparatus. It is a top view of a correction filter. It is sectional drawing of a correction filter. It is a perspective view for demonstrating the mode of the light on a photodetector. It is a schematic sectional drawing of a compound objective lens. It is sectional drawing of another embodiment of the optical head apparatus provided with the compound objective lens. It is a figure which shows the manufacturing method of a correction filter. It is a figure which shows another manufacturing method of a correction filter.

Explanation of symbols

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

Claims (4)

An optical head device that records or reproduces information on a first information medium having a substrate having a first thickness and a second information medium having a substrate having a second thickness,
A light source;
An objective lens for receiving light emitted from the light source and condensing the light on the information recording surface of the first information medium or the second information medium;
A photodetector that receives the light reflected by the first information medium or the second information medium and converts it into an electrical signal;
The objective lens is on the information recording surface of the information medium having a substrate, a objective lens for collecting the light through the substrate, comprising a refraction lens, and a hologram lens having an aberration correction action ,
The hologram lens comprises a plurality of regions,
The plurality of regions include a first region and a second region closer to the optical axis of the objective lens than the first region,
The hologram lens is integrally formed on a surface having a larger curvature of the refractive lens, that is, a surface having a smaller radius of curvature,
Both the light that has passed through the first area and the light that has passed through the second area have the first thickness on the information recording surface of the first information medium having the substrate having the first thickness. Condensing through the substrate,
Only the light that has passed through the second region is condensed on the information recording surface of the second information medium having the second thickness substrate through the second thickness substrate,
The first thickness is smaller than the second thickness,
The numerical aperture for the first light through the substrate thickness is collecting light, the light through the second thickness substrate rather large compared to the numerical aperture for focusing the light,
The light source is a semiconductor laser having an astigmatic difference,
In the optical path from the light source to the objective lens, a light intensity correction element is provided,
2. The optical head device according to claim 1, wherein the light intensity correction element includes a liquid crystal cell, and the liquid crystal cell reduces aberration . The optical head device according to claim 1, wherein the hologram lens has a concentric relief shape. The light emitted from the light source,
When the light is condensed on the information recording surface of the first information medium having the substrate having the first thickness through the substrate having the first thickness;
When the light is condensed on the information recording surface of the second information medium having the second thickness substrate through the second thickness substrate;
3. The optical head device according to claim 1, wherein light is received by a common photodetector and converted into an electric signal in any case. And the first information medium having a first thickness substrate, claims 1 to 3 for recording or reproducing information for the second of the second information medium having a thick substrate Using any one of the optical head devices,
Condensing the light emitted from the light source on the information recording surface of the first information medium or the second information medium;
A tracking error signal is detected by receiving light reflected from the first information medium or the second information medium by the photodetector;
An optical disc apparatus for reproducing information from the first information medium or the second information medium.

Images