Patent Publication Number: US-7595472-B2

Title: Optical head device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of priority from and is a Divisional application of application Ser. No. 11/388,616 filed Mar. 24, 2006, which is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-199140, filed Jul. 7, 2005, the entire contents of both applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical head device which reproduces information recorded in a multilayered optical disk. 
   2. Description of the Related Art 
   In a situation in which an infrastructure capable of enjoying digital information of a dynamic image increasingly upgrades and expands such as satellite digital broadcasting or ground digital broadcasting, there is a demand for realization of a conveyable recording medium capable of recording a larger capacity of higher-definition digital information. To meet this demand, there are developed various types of next-generation high-density optical disks each having a recording capacity three or four times that of a presently spreading digital versatile disk (DVD). Among them, there is developed an HD DVD using a blue purple laser diode having a 405 nm wavelength band and an objective lens having an optimized substrate thickness of 0.6 mm and a numerical aperture of 0.65, from viewpoints of compatibility with the existing compact disk (CD) and DVD, ease of realizing a thin optical head device for a notebook-size personal computer, a low drive manufacturing cost, and a low disk manufacturing cost. In the HD DVD, development of a double-layer disk including double recording layers is advanced in parallel in the same manner as in the DVD for a purpose of further increasing a recording information amount. 
   The substrate thickness of the double-layer disk deviates from that of a single-layer disk, which is 0.6 mm. Therefore, a wave aberration is generated in a beam spot, and optical characteristics deteriorate. To suppress the wave aberration to an allowable value or less in the double-layer disk of the existing DVD, an interlayer thickness between two recording layers is defined as 55 μm±15 μm. When aberration standards similar to those of the DVD are applied to the HD-DVD, the interlayer thickness between two layers is reduced to about 25 μm. This is because the wave aberration generated with respect to an error of the substrate thickness is substantially proportional to the fourth power of the objective lens numerical aperture, and inversely proportional to a laser wavelength. Since the interlayer thickness of the HD DVD becomes smaller than that of the DVD in this manner, there is a remarkable influence of a phenomenon where undesired light reflected by a non-reproduction layer of two recording layers leaks to and falls on a photodetector, which is a so-called interlayer crosstalk. That is, since the interlayer thickness is small in the HD DVD, a beam diameter of the leak light from the non-reproduction layer on the face of the photodetector is smaller than that of the DVD. Therefore, a quantity of leak light increases. This is a cause for deterioration of a reproduction signal. Therefore, in addition to defining of the interlayer thickness, an interlayer crosstalk reducing measure is necessary in the HD DVD. 
   Jpn. Pat. Appln. KOKAI Publication No. 2003-323736 discloses an interlayer crosstalk reducing method due to a pinhole element with reference to, for example,  FIG. 1 . A position of the pinhole element is adjusted to pass signal light from a reproduction layer. Since the beam diameter of the leak light from the non-reproduction layer increases on the pinhole element, a large part of the leak light is interrupted by the pinhole element. That is, the signal light can pass through the pinhole element, but a large part of the leak light cannot pass through the pinhole element. As a result, the interlayer crosstalk is effectively reduced. The position of the pinhole element needs to be mechanically adjusted with good precision with respect to three axes, that is, an optical axis (z-axis) and two axes (x-axis and y-axis) crossing each other at right angles in a plane perpendicular to the optical axis. Therefore, the pinhole element is usable in an experimental level, but it is substantially difficult to mount the element on a product. 
   Jpn. Pat. Appln. KOKAI Publication No. 6-180851 discloses that a liquid crystal shutter whose aperture size and position are adjustable is disposed in a focal plane of a lens for condensing reflected light from an optical information recording medium or near the focal plane with reference to, for example,  FIG. 1 . This liquid crystal shutter is disposed for a purpose of applying to the photodetector a light beam having a diameter which is smaller than a diffraction limit diameter of the condensing lens, and the shutter is not used for the reduction of the interlayer crosstalk. 
   BRIEF SUMMARY OF THE INVENTION 
   An optical head device according to an aspect of the present invention comprises a light source which emits a light beam, an objective lens which condenses the light beam on an optical disk, a condensing lens which condenses reflected light from the optical disk, a liquid crystal panel disposed near a focal point of the condensing lens and having pixels, a driver which drives the pixels of the liquid crystal panel to pass the reflected light from a reproduction layer of the optical disk and partially interrupt the reflected light from a non-reproduction layer of the optical disk, and a photodetector which detects light passed through the liquid crystal panel. 
   An optical head device according to another aspect of the present invention comprises a light source which emits a light beam, an objective lens which condenses the light beam on an optical disk, a condensing lens which condenses reflected light from the optical disk, a shielding element disposed near a focal point of the condensing lens and having a pinhole, and a photodetector which detects light passed through the pinhole of the shielding element. The light source is capable of selectively emit a high-output light beam and a low-output light beam. The shielding element includes a transparent substrate and a thin film disposed on the transparent substrate. The thin film is partially melted by application of the high-output light beam to form the pinhole. The pinhole passes the reflected light from a reproduction layer of the optical disk and partially interrupts the reflected light from a non-reproduction layer of the optical disk. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  shows an optical head device in a first embodiment of the present invention; 
       FIG. 2  shows details of a liquid crystal panel shown in  FIG. 1 ; 
       FIG. 3  shows a relation between a pinhole diameter of the liquid crystal panel and interlayer crosstalk; 
       FIG. 4  shows a method of adjusting a position of the liquid crystal panel shown in  FIG. 1 ; 
       FIG. 5  shows details of another liquid crystal panel shown in  FIG. 1 ; 
       FIG. 6  shows an optical head device in a second embodiment of the present invention; 
       FIG. 7  shows details of a liquid crystal panel shown in  FIG. 6 ; 
       FIG. 8  shows details of a multilayered liquid crystal panel for use in an optical head device in a third embodiment of the present invention; 
       FIG. 9  shows a driven state of the multilayered liquid crystal panel shown in  FIG. 8 ; 
       FIG. 10  shows another driven state of the multilayered liquid crystal panel shown in  FIG. 8 ; 
       FIGS. 11A and 11B  show a shielding element in an optical head device in a fourth embodiment of the present invention; and 
       FIG. 12  shows a method of adjusting a position of the shielding element shown in  FIGS. 11A and 11B . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described hereinafter in detail with reference to the drawings. 
   First Embodiment 
     FIG. 1  shows an optical head device in a first embodiment of the present invention. In  FIG. 1 , an optical disk  100  is a double-layer optical disk including two recording layers  101 A and  101 B each provided with, for example, spiral groove and land regions. The recording layer  101 A nearer to an incidence side of a light beam is referred to as a 0-th recording layer, and the recording layer  101 B farther from the incidence side of the light beam is referred to as a first recording layer. The optical disk  100  is rotated by a spindle motor  102  during recording and reproducing. 
   The optical head device includes a laser light source  103  which emits a light beam, a collimator lens  104  which changes the light beam from the laser light source  103  into a parallel light beam, an objective lens  108  which condenses the light beam onto the optical disk  100 , and a polarization beam splitter  105  which separates light directed to the optical disk  100  and light reflected from the optical disk  100 . The optical head device further includes a raising mirror  106  and a ¼ wavelength plate  107  on an optical path between the polarization beam splitter  105  and the objective lens  108 . The optical head device includes a beam splitter  109  which splits the reflected light beam from the optical disk  100  on an optical path of the reflected light returning from the optical disk  100  and split by the polarization beam splitter  105 . The optical head device further includes a first condensing lens  110  which condenses the reflected light from the optical disk  100 , a cylindrical lens  111  which gives an astigmatism to the reflected light, and a first photodetector  112  having light receiving regions on a transmission optical path of the polarization beam splitter  105 . On a reflection optical path of the polarization beam splitter  105 , the device includes a second condensing lens  113  which condenses the reflected light from the optical disk  100 , a liquid crystal panel  114  disposed near a focal point of the second condensing lens  113  and having pixels, and a photodetector  115  which detects light passed through the liquid crystal panel  114 . 
   During recording of information in the 0-th recording layer  101 A or the first recording layer  101 B, or reproducing of the information from the 0-th recording layer  101 A or the first recording layer  101 B, the light beam is focused on a recording layer as a recording object or a recording layer (reproduction layer) as a reproduction object by a light application unit including a laser light source  103  such as a laser diode, the collimator lens  104 , the polarization beam splitter  105 , the raising mirror  106 , the ¼ wavelength plate  107 , and the objective lens  108 . 
   The reproducing will be described. The beam of linearly polarized light emitted from the laser light source  103  is converted from a diverging light beam into a parallel light beam by the collimator lens  104 , then passes through the polarization beam splitter  105 , and is further reflected by the raising mirror  106  to enter the ¼ wavelength plate  107 . The parallel light beam applied to the ¼ wavelength plate  107  is converted into circularly polarized light, and focused on (one of reflection layers  101 C and  101 D of) the reproduction layer (one of the recording layers  101 A and  101 B) of the optical disk  100  by the objective lens  108 . 
   The light reflected by the reproduction layer is converted into linearly polarized light perpendicular to the incoming linearly polarized light through the objective lens  108  and the ¼ wavelength plate  107  in reverse to the incident light, and then reflected by the raising mirror  106  to enter the polarization beam splitter  105 . The polarization beam splitter  105  reflects the light beam applied from the raising mirror  106  to bring it to the beam splitter  109 . The beam splitter  109  splits the applied light beam into a parallel light beam for focusing control and a parallel light beam for both tracking control and reproducing in accordance with a predetermined light quantity ratio. 
   The parallel light beam for focusing control, transmitted through the beam splitter  109 , is converted into a convergent light beam by the first condensing lens  110 , and further refracted by and passed through the cylindrical lens  111 . Thereafter, the light beam is condensed onto the first photodetector  112 . The light beam on a light receiving face of the first photodetector  112  is formed into an elliptical shape, if there is a focal error. Moreover, directions of long and short axes of the ellipse change depending on a direction of focal deviation. By use of this, a focal error signal is generated by a focal error detection calculator  121 . The objective lens  108  is driven in a direction perpendicular to faces of the recording layers  101 A and  101 B by a lens actuator  116  based on this focal error detection signal. Accordingly, the light beam is correctly focused on the reproduction layer. Here, as a focal error detection method, a typical astigmatism method has been described, but the focal error detection method is not limited to this method, and a method such as a knife edge method or a beam size method may be used. 
   On the other hand, the parallel light beam for both the tracking control and the reproducing, reflected by the beam splitter  109 , is converted into a convergent light beam by the second condensing lens  113 , and then passes through the liquid crystal panel  114 , disposed near the focal point of the condensing lens  113  and having pixels, to enter the second photodetector  115 . The liquid crystal panel  114  is driven by a liquid crystal driver  117 . The second photodetector  115  is a multi-segment detector having, for example, four light receiving faces, each of which generates an output signal depending on incident light quantity. Output signals respectively corresponding to the light receiving faces of the second photodetector  115  are subjected to a known calculation equation in a calculator  122  to thereby generate a reproduction signal and a tracking error signal. The objective lens  108  is driven in in-plane directions of the recording layers  101 A and  101 B by the lens actuator  116  based on the tracking error signal, and the light beam are positioned in a target track on the reproduction layer. 
   The liquid crystal panel  114  has square pixels  204  as shown in, for example,  FIG. 2 . The pixels  204  are selectively brought into a transmitted state by the liquid crystal driver  117 . For example, the pixels  204  are voltage-driven by the liquid crystal driver  117 , brought into the transmitted state during application of a voltage, and brought into a non-transmitted state in a case where any voltage is not applied. 
   Signal light  201  as the reflected light from the reproduction layer of the optical disk  100 , and the reflected light from the non-reproduction layer, that is, leak light  202  enter the liquid crystal panel  114 . A beam diameter of the leak light  202  is larger than that of the signal light  201  on the liquid crystal panel  114 . Therefore, among the pixels  204  shown in  FIG. 2 , pixels irradiated with the signal light  201  and shown in white are brought into the transmitted state to form a pinhole  205 , and the other pixels of regions shown by dots are brought into the non-transmitted state. As a result, a large part of the leak light  202  is interrupted by the liquid crystal panel  114 , but the signal light  201  passes through the pinhole  205 , and therefore interlayer crosstalk is largely reduced. 
     FIG. 3  shows an analysis result of an interlayer crosstalk amount (quantity of leak light to the light receiving face of the second photodetector  115 ) with respect to a pinhole diameter (diameter of the pinhole  205 ). Analysis conditions were set to an interlayer thickness of 25 μm between the recording layers  101 A and  101 B of the optical disk  100 , an optical system magnification of eight times, and a light receiving face size of 100 μm×100 μm of the second photodetector  115 . In  FIG. 3 , the ordinate indicates a value obtained by standardizing a leak amount in a case where the pinhole  205  is disposed by use of a leak light quantity in a case where any pinhole  205  is not disposed. As seen from  FIG. 3 , when the pinhole diameter is set to 200 μm or less, the interlayer crosstalk is reduced to ½ or less. To set the pinhole diameter to 200 μm or less, it is preferable to set a pixel size of the liquid crystal panel  114  to about 20 μm or less. 
   Next, there will be described a method of adjusting a position of the liquid crystal panel  114  with reference to  FIG. 4 . The method is performed during adjustment in assembling the optical head device shown in  FIG. 1 . The position of the liquid crystal panel  114  is adjusted in a procedure of the following steps S 1  to S 6 . 
   &lt;Step S 1 &gt; 
   A mirror disk  301  for adjustment is set instead of the optical disk  100 , and the objective lens  108  is focused on the surface of the mirror disk  301  for adjustment by the lens actuator  116 . A single-layer disk having a flat light reflecting face having a high reflectance may be used in the mirror disk  301  for adjustment. 
   &lt;Step S 2 &gt; 
   A mirror  302  for adjustment is disposed on the back of the liquid crystal panel  114 . 
   &lt;Step S 3 &gt; 
   All of the pixels  204  of the liquid crystal panel  114  are brought into transmitted states. 
   &lt;Step S 4 &gt; 
   The light transmitted through the liquid crystal panel  114 , reflected by the mirror  302  for adjustment, and transmitted through the beam splitter  109  is caused to enter a collimation tester  303  for adjustment. 
   &lt;Step S 5 &gt; 
   While observing a convergent/divergent state of the incidence light by the collimation tester  303  for adjustment, the position of the liquid crystal panel  114  along the optical axis is adjusted so that the incidence light forms a parallel light beam. It is to be noted that since the second condensing lens  113  usually has a numerical aperture of about 0.06 to 0.08, the condensing lens  113  has a focal depth of about 30 μm to 60 μm. Therefore, the adjustment of the position of the liquid crystal panel  114  along the optical axis is not a difficult operation. 
   &lt;Step S 6 &gt; 
   The mirror  302  for adjustment is removed. While monitoring the photodetector  115  so that an incident light quantity does not decrease, pixels to which any light is not applied are turned off, and pixels to which the light is applied are turned on by the liquid crystal driver  117  among the pixels  204  of the liquid crystal panel  114  to form the pinhole  205 . 
   The pinhole  205  is formed using the liquid crystal panel  114  in this manner, and this provides an advantage that mechanical adjustment in an in-plane direction perpendicular to the optical axis, which is usually necessary for pinholes, is obviated or simplified. 
   The liquid crystal panel  114  shown in  FIG. 2  has the square pixels  204 , the shapes of the pixels are not limited to them, and the panel may have honeycomb-shaped pixels  206  as shown in, for example,  FIG. 5 . Even in this case, the pixels of the regions shown in white are brought into the transmitted states by the liquid crystal driver  117  to form the pinhole  205 . 
   Second Embodiment 
   In a second embodiment of the present invention, as shown in  FIG. 6 , an optical head device has a constitution in which a diffraction grating  118  for three beams is added to the optical head device shown in  FIG. 1 . In general, in the optical head device, a light beam emitted from a light source are often split into multiple beams in order to generate a reproduction signal, a focal error signal, a tracking error signal and the like.  FIG. 6  shows an example in which three light beams are formed in order to generate the tracking error signal by a differential push-pull (DPP) method. 
   A liquid crystal panel  114  is capable of easily handling such multiple beams. That is, as shown in  FIG. 7 , the liquid crystal panel  114  interrupts a large part of leak light  202 , and it is possible to easily form three pinholes  205  which pass three beams as signal light  201  from a reproduction layer. As to a usual pinhole element provided with physical holes, it is difficult to adjust relative positions of pinholes in accordance with a solid difference of the optical head device. Therefore, it is remarkably difficult to simultaneously transmit beams through pinholes in the usual pinhole provided with the physical holes. However, this is easily possible by the liquid crystal panel  114  in the present embodiment. 
   During adjustment in assembling the optical head device of the present embodiment, a position of the liquid crystal panel  114  is basically adjusted by a method described above with reference to  FIG. 4 . The present embodiment is different from the second embodiment in that three light beams enter the liquid crystal panel  114 , but the position of the panel may be adjusted using a main beam (0-order diffracted light from the diffraction grating  118  for three beams). That is, a collimation tester  303  may be disposed in a far position to such an extent that three beams are physically split, and the above procedure of steps S 1  to S 6  may be performed. 
   The second embodiment shows an example of the optical head device using three beams, but it is effective to form pinholes by the liquid crystal panel  114  even in an optical head device using two beams or four or more beams. 
   Third Embodiment 
   In a third embodiment of the present invention, an optical head device includes a multilayered liquid crystal panel  124  shown in  FIG. 8  instead of the liquid crystal panel  114  for use in the second embodiment. The multilayered liquid crystal panel  124  includes unit panels  123  each of which is similar to the liquid crystal panel  114  described above, and these panels are laminated along an optical axis. Another constitution of the optical head device is similar to that of the second embodiment. A method of adjusting a position of the multilayered liquid crystal panel  124  along an optical axis during adjustment in assembling the optical head device is basically as described with reference to  FIG. 4 , but a treatment of the above-described step S 6  is performed as follows in detail. 
   That is, among pixels  204  of the unit panels  123 , pixels to which any light is not applied are turned off, and pixels to which the light is applied are turned on by a liquid crystal driver  117  to form pinholes  205  in the same manner as in the step S 6 . However, as shown in  FIG. 9 , the pixels  204  of the unit panel  123  of each layer of the multilayered liquid crystal panel  124  are turned on/off so that the pinhole  205  is formed near a focal point of a second condensing lens  113 , especially in a focal point of signal light  201 . In this case, there are advantages that margins in adjusting the position of the multilayered liquid crystal panel  124  along the optical axis increase and that the position adjustment is further facilitated. 
   During the laminating of the unit panels  123  in the multilayered liquid crystal panel  124 , as shown in, for example,  FIG. 10 , the pixels  204  of one of the unit panels  123  may partially overlap with the pixels  204  of another one of the unit panels  123  in projection onto a plane perpendicular to the optical axis. This prevents light from being leaked between the pixels  204 . 
   Fourth Embodiment 
   In a fourth embodiment of the present invention, an optical head device has a shielding element  401  shown in  FIGS. 11A and 11B  instead of the liquid crystal panel. The laser light source  103  is capable of selectively emitting a low-output light beam for use in recording and reproducing information and a high-output light beam having a higher energy density. Another constitution of the optical head device is similar to that of the first embodiment shown in  FIG. 1 . The shielding element  401  is disposed near a focal point of a condensing lens  113 , and includes a transparent substrate  402  and a thin film  403  such as a Cr film deposited or formed by sputtering, plating or the like on the transparent substrate  402 . As shown in  FIG. 11B , a pinhole  404  is formed in the thin film  403 . 
   Next, there will be described a method of adjusting the shielding element  401  shown in  FIGS. 11A and 11B  during adjustment in assembling the optical head device with reference to  FIG. 12 . A position of the shielding element  401  is adjusted by a procedure of the following steps S 11  to S 14 , and the pinhole  404  is formed. 
   &lt;Step S 11 &gt; 
   A mirror disk  301  for adjustment is set instead of the optical disk  100 , and an objective lens  108  is focused on the surface of the mirror disk  301  for adjustment by a lens actuator  116 . A single-layer disk having a flat light reflecting face having a high reflectance may be used in the mirror disk  301  for adjustment. 
   &lt;Step S 12 &gt; 
   The light reflected by the shielding element  401  and transmitted through a beam splitter  109  is caused to enter a collimation tester  303  for adjustment. 
   &lt;Step S 13 &gt; 
   While observing a convergent/divergent state of incidence light by the collimation tester  303  for adjustment, the position of the shielding element  401  along the optical axis is adjusted so that the incidence light forms a parallel light beam. As a result, the thin film  403  of the shielding element  401  is disposed in a focal point of reflected light from the mirror disk  301  for adjustment, that is, a focal point of reflected light from a reproduction face of the optical disk  100 . It is to be noted that since the second condensing lens  113  usually has a numerical aperture of about 0.06 to 0.08, the condensing lens  113  has a focal depth of about 30 μm to 60 μm. Therefore, the adjustment of the position of the shielding element  401  along the optical axis is not a difficult operation. 
   &lt;Step S 14 &gt; 
   A high-output light beam is output from the laser light source  103 , and this high-output light beam is applied to the thin film  403  of the shielding element  401  to melt the film and form the pinhole  404  as shown in  FIG. 11B . 
   The thin film  403  has, for example, such a film thickness that the film melts under the application of the high-output light beam in the step S 14 , that is, in a state in which a laser energy density is high, but does not melt under the application of the low-output light beam for use during the recording and the reproducing of the optical disk. Alternatively, the thin film  403  may be made of a material which melts under the application of the high-output light beam in the step S 14  but does not melt under the application of the low-output light beam for use during the recording and the reproducing of the usual optical disk. 
   The shielding element  401  provided with the pinhole  404  in the steps S 11  to S 14  interrupts a large part of leak light from the non-reproduction layer during the reproducing of the information from the double-layer disk. On the other hand, the element transmits the signal light from the reproduction layer through the pinhole  404 . 
   According to the fourth embodiment, interlayer crosstalk can be effectively reduced without requiring any mechanical adjustment of the shielding element  401  in an in-plane direction which is perpendicular to the optical axis. 
   It is to be noted that the present invention is not limited to the above embodiments as such, and constituting elements can be modified and embodied in an implementation stage within a range that does not depart from the scope. Various inventions can be formed by appropriately combining constituting elements disclosed in the above embodiments. For example, several constituting elements may be deleted from all of the constituting elements described in the embodiments. Furthermore, the constituting elements ranging over different embodiments may be appropriately combined. For example, the shielding element of the fourth embodiment may be combined with the optical head device using multiple beams as described in the second embodiment. 
   For example, there has been described an example in which the single-layer disk is used in the mirror disk  301  for adjustment in the above embodiments, but, for example, a multilayered disk having flat reflection faces may be used. In this case, reflected light from one of the reflection faces may be used. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general invention concept as defined by the appended claims and their equivalents.