Optical head device

An optical head device includes a light source which emits a light beam, an objective lens which condenses the light beam on the 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.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application 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 which 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.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described hereinafter in detail with reference to the drawings.

FIG. 1shows an optical head device in a first embodiment of the present invention. InFIG. 1, an optical disk100is a double-layer optical disk including two recording layers101A and101B each provided with, for example, spiral groove and land regions. The recording layer101A nearer to an incidence side of a light beam is referred to as a 0-th recording layer, and the recording layer101B farther from the incidence side of the light beam is referred to as a first recording layer. The optical disk100is rotated by a spindle motor102during recording and reproducing.

The optical head device includes a laser light source103which emits a light beam, a collimator lens104which changes the light beam from the laser light source103into a parallel light beam, an objective lens108which condenses the light beam onto the optical disk100, and a polarization beam splitter105which separates light directed to the optical disk100and light reflected from the optical disk100. The optical head device further includes a raising mirror106and a ¼ wavelength plate107on an optical path between the polarization beam splitter105and the objective lens108. The optical head device includes a beam splitter109which splits the reflected light beam from the optical disk100on an optical path of the reflected light returning from the optical disk100and split by the polarization beam splitter105. The optical head device further includes a first condensing lens110which condenses the reflected light from the optical disk100, a cylindrical lens111which gives an astigmatism to the reflected light, and a first photodetector112having light receiving regions on a transmission optical path of the polarization beam splitter105. On a reflection optical path of the polarization beam splitter105, the device includes a second condensing lens113which condenses the reflected light from the optical disk100, a liquid crystal panel114disposed near a focal point of the second condensing lens113and having pixels, and a photodetector115which detects light passed through the liquid crystal panel114.

During recording of information in the 0-th recording layer101A or the first recording layer101B, or reproducing of the information from the 0-th recording layer101A or the first recording layer101B, 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 source103such as a laser diode, the collimator lens104, the polarization beam splitter105, the raising mirror106, the ¼ wavelength plate107, and the objective lens108.

The reproducing will be described. The beam of linearly polarized light emitted from the laser light source103is converted from a diverging light beam into a parallel light beam by the collimator lens104, then passes through the polarization beam splitter105, and is further reflected by the raising mirror106to enter the ¼ wavelength plate107. The parallel light beam applied to the ¼ wavelength plate107is converted into circularly polarized light, and focused on (one of reflection layers101C and101D of) the reproduction layer (one of the recording layers101A and101B) of the optical disk100by the objective lens108.

The light reflected by the reproduction layer is converted into linearly polarized light perpendicular to the incoming linearly polarized light through the objective lens108and the ¼ wavelength plate107in reverse to the incident light, and then reflected by the raising mirror106to enter the polarization beam splitter105. The polarization beam splitter105reflects the light beam applied from the raising mirror106to bring it to the beam splitter109. The beam splitter109splits 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 splitter109, is converted into a convergent light beam by the first condensing lens110, and further refracted by and passed through the cylindrical lens111. Thereafter, the light beam is condensed onto the first photodetector112. The light beam on a light receiving face of the first photodetector112is 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 calculator121. The objective lens108is driven in a direction perpendicular to faces of the recording layers101A and101B by a lens actuator116based 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 splitter109, is converted into a convergent light beam by the second condensing lens113, and then passes through the liquid crystal panel114, disposed near the focal point of the condensing lens113and having pixels, to enter the second photodetector115. The liquid crystal panel114is driven by a liquid crystal driver117. The second photodetector115is 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 photodetector115are subjected to a known calculation equation in a calculator122to thereby generate a reproduction signal and a tracking error signal. The objective lens108is driven in in-plane directions of the recording layers101A and101B by the lens actuator116based on the tracking error signal, and the light beam are positioned in a target track on the reproduction layer.

The liquid crystal panel114has square pixels204as shown in, for example,FIG. 2. The pixels204are selectively brought into a transmitted state by the liquid crystal driver117. For example, the pixels204are voltage-driven by the liquid crystal driver117, 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 light201as the reflected light from the reproduction layer of the optical disk100, and the reflected light from the non-reproduction layer, that is, leak light202enter the liquid crystal panel114. A beam diameter of the leak light202is larger than that of the signal light201on the liquid crystal panel114. Therefore, among the pixels204shown inFIG. 2, pixels irradiated with the signal light201and shown in white are brought into the transmitted state to form a pinhole205, 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 light202is interrupted by the liquid crystal panel114, but the signal light201passes through the pinhole205, and therefore interlayer crosstalk is largely reduced.

FIG. 3shows an analysis result of an interlayer crosstalk amount (quantity of leak light to the light receiving face of the second photodetector115) with respect to a pinhole diameter (diameter of the pinhole205). Analysis conditions were set to an interlayer thickness of 25 μm between the recording layers101A and101B of the optical disk100, an optical system magnification of eight times, and a light receiving face size of 100 μm×100 μm of the second photodetector115. InFIG. 3, the ordinate indicates a value obtained by standardizing a leak amount in a case where the pinhole205is disposed by use of a leak light quantity in a case where any pinhole205is not disposed. As seen fromFIG. 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 panel114to about 20 μm or less.

Next, there will be described a method of adjusting a position of the liquid crystal panel114with reference toFIG. 4. The method is performed during adjustment in assembling the optical head device shown inFIG. 1. The position of the liquid crystal panel114is adjusted in a procedure of the following steps S1to S6.

A mirror disk301for adjustment is set instead of the optical disk100, and the objective lens108is focused on the surface of the mirror disk301for adjustment by the lens actuator116. A single-layer disk having a flat light reflecting face having a high reflectance may be used in the mirror disk301for adjustment.

A mirror302for adjustment is disposed on the back of the liquid crystal panel114.

All of the pixels204of the liquid crystal panel114are brought into transmitted states.

The light transmitted through the liquid crystal panel114, reflected by the mirror302for adjustment, and transmitted through the beam splitter109is caused to enter a collimation tester303for adjustment.

While observing a convergent/divergent state of the incidence light by the collimation tester303for adjustment, the position of the liquid crystal panel114along 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 lens113usually has a numerical aperture of about 0.06 to 0.08, the condensing lens113has a focal depth of about 30 μm to 60 μm. Therefore, the adjustment of the position of the liquid crystal panel114along the optical axis is not a difficult operation.

The mirror302for adjustment is removed. While monitoring the photodetector115so 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 driver117among the pixels204of the liquid crystal panel114to form the pinhole205.

The pinhole205is formed using the liquid crystal panel114in 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 panel114shown inFIG. 2has the square pixels204, the shapes of the pixels are not limited to them, and the panel may have honeycomb-shaped pixels206as 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 driver117to form the pinhole205.

In a second embodiment of the present invention, as shown inFIG. 6, an optical head device has a constitution in which a diffraction grating118for three beams is added to the optical head device shown inFIG. 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. 6shows 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 panel114is capable of easily handling such multiple beams. That is, as shown inFIG. 7, the liquid crystal panel114interrupts a large part of leak light202, and it is possible to easily form three pinholes205which pass three beams as signal light201from 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 panel114in the present embodiment.

During adjustment in assembling the optical head device of the present embodiment, a position of the liquid crystal panel114is basically adjusted by a method described above with reference toFIG. 4. The present embodiment is different from the second embodiment in that three light beams enter the liquid crystal panel114, but the position of the panel may be adjusted using a main beam (0-order diffracted light from the diffraction grating118for three beams). That is, a collimation tester303may be disposed in a far position to such an extent that three beams are physically split, and the above procedure of steps S1to S6may 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 panel114even in an optical head device using two beams or four or more beams.

In a third embodiment of the present invention, an optical head device includes a multilayered liquid crystal panel124shown inFIG. 8instead of the liquid crystal panel114for use in the second embodiment. The multilayered liquid crystal panel124includes unit panels123each of which is similar to the liquid crystal panel114described 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 panel124along an optical axis during adjustment in assembling the optical head device is basically as described with reference toFIG. 4, but a treatment of the above-described step S6is performed as follows in detail.

That is, among pixels204of the unit panels123, 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 driver117to form pinholes205in the same manner as in the step S6. However, as shown inFIG. 9, the pixels204of the unit panel123of each layer of the multilayered liquid crystal panel124are turned on/off so that the pinhole205is formed near a focal point of a second condensing lens113, especially in a focal point of signal light201. In this case, there are advantages that margins in adjusting the position of the multilayered liquid crystal panel124along the optical axis increase and that the position adjustment is further facilitated.

During the laminating of the unit panels123in the multilayered liquid crystal panel124, as shown in, for example,FIG. 10, the pixels204of one of the unit panels123may partially overlap with the pixels204of another one of the unit panels123in projection onto a plane perpendicular to the optical axis. This prevents light from being leaked between the pixels204.

In a fourth embodiment of the present invention, an optical head device has a shielding element401shown inFIGS. 11A and 11Binstead of the liquid crystal panel. The laser light source103is 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 inFIG. 1. The shielding element401is disposed near a focal point of a condensing lens113, and includes a transparent substrate402and a thin film403such as a Cr film deposited or formed by sputtering, plating or the like on the transparent substrate402. As shown inFIG. 11B, a pinhole404is formed in the thin film403.

Next, there will be described a method of adjusting the shielding element401shown inFIGS. 11A and 11Bduring adjustment in assembling the optical head device with reference toFIG. 12. A position of the shielding element401is adjusted by a procedure of the following steps S11to S14, and the pinhole404is formed.

A mirror disk301for adjustment is set instead of the optical disk100, and an objective lens108is focused on the surface of the mirror disk301for adjustment by a lens actuator116. A single-layer disk having a flat light reflecting face having a high reflectance may be used in the mirror disk301for adjustment.

The light reflected by the shielding element401and transmitted through a beam splitter109is caused to enter a collimation tester303for adjustment.

While observing a convergent/divergent state of incidence light by the collimation tester303for adjustment, the position of the shielding element401along the optical axis is adjusted so that the incidence light forms a parallel light beam. As a result, the thin film403of the shielding element401is disposed in a focal point of reflected light from the mirror disk301for adjustment, that is, a focal point of reflected light from a reproduction face of the optical disk100. It is to be noted that since the second condensing lens113usually has a numerical aperture of about 0.06 to 0.08, the condensing lens113has a focal depth of about 30 μm to 60 μm. Therefore, the adjustment of the position of the shielding element401along the optical axis is not a difficult operation.

A high-output light beam is output from the laser light source103, and this high-output light beam is applied to the thin film403of the shielding element401to melt the film and form the pinhole404as shown inFIG. 11B.

The thin film403has, for example, such a film thickness that the film melts under the application of the high-output light beam in the step S14, 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 film403may be made of a material which melts under the application of the high-output light beam in the step S14but 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 element401provided with the pinhole404in the steps S11to S14interrupts 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 pinhole404.

According to the fourth embodiment, interlayer crosstalk can be effectively reduced without requiring any mechanical adjustment of the shielding element401in 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 disk301for 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.