Patent Application
20090219795
The present invention relates to an optical disc drive for recording or reproducing data by means of a minute optical spot, and an optical head for use in the optical disc drive.
In the field of the optical discs that perform recording/reproducing data by means of a minute optical spot, there have been wide-spread of ROM (Read Only Memory) medium, on which an emboss-like data pit train is formed for allowing reproduction only, followed by prevalence of CD-R (Compact Disc-Recordable) and DVD-R (Digital Versatile Disc-Recordable), which allow recording of data in addition. Optical discs referred to as CD-RW (CD-ReWritable) and DVD-RW (DVD-ReWritable) have appeared on the market. Further, as a next-generation DVD, a standard referred to as HD DVD that uses a blue light source is also issued.
Recently, among those optical discs as described above, an optical disc is developed and produced commercially which achieves a larger recording capacity by forming two recording layers that receive light from the same substrate-incident-surface for recording or reproducing data. As a type of the DVD-ROM, an optical disc including two recording layers is released for use in a long-time movie etc. Also as the DVD-R, a medium including two recording layers is standardized and has become used. Similarly, the standard of the two recording layers has also been set for the next-generation HD DVD. However, since the spacing between the two layers is smaller than that of the DVD, it is necessary to solve a new problem involved therein.
On the other hand, as shown in the right-hand side in
It is assumed here that the photodetector 12 is disposed in the vicinity of the position of the condensed spot of the signal light generated by the objective lens 17. In this case, the ratio (received light ratio) of the amount of light received by the photodetector 12 among the cross-talk light from the adjacent layer to the amount of the received signal is almost the same order irrespective of the case where which of the layers is accessed. Here, if the layer spacing is being reduced, the virtual focal point of the cross-talk light approaches the condensed spot of the signal light, whereby the difference between both the optical paths is reduced. As a result the beam diameter of the cross-talk light is reduced on the photodetector, and thus the received light ratio of the cross-talk light is relatively increased.
When the cross-talk light is received, not only the DC component of the received signal light increases, but also the fluctuation component thereof also occurs. This is because outputs from a single laser light source return from the reflecting surfaces along different travel distances having therebetween a difference corresponding to the layer spacing, and the outputs are overlapped each other on the photodetector 12 to generate an optical interference caused by the difference in the travel distance. If the layer spacing does not vary, the condition of the interference is based on the constant difference in the travel distance. However, since the layer spacing varies delicately with the location in an actual optical disc, the condition of the interference varies when the condensed spot moves along the track direction. For example, if the phase difference between the two interfering reflected lights varies from zero to π, that is, if the optical length of the layer spacing which provides the distance difference varies by an amount of ¼ wavelength, the interference condition changes from a condition under which the outputs intensify each other most strongly to a condition under which the outputs weaken each other most strongly. Assuming that the amount of the received signal light and the amount of the received cross-talk are Is and IC, respectively, the total amount of received light changes from Is+Ic to Is−Ic. It is to be noted that Is and Ic are the quantity representing the electric field intensity of the received light, and that the received power is a quantity proportional to the square thereof. It is a matter of course that the signal light and the cross-talk light do not completely overlap each other in the receiving area of the photodetector, and that the phase of each light within the section thereof is not uniform and may involve a disorder, and thus the above case is the worst condition.
If there is a larger amount of received cross-talk light, the DC component of the received light of the photodetector changes with a movement of the optical spot along the track direction, as shown in
In the conventional two-layer medium having a larger layer spacing, since the amount of received cross-talk light is small, the range of variation can be neglected even if variation occurs in the interference. However, along with a reduction in the layer spacing, the degradation in the reproducing characteristic causes a problem that is hardly neglected.
The influence may be critical if the condensed spot is formed from a sub-beam in the optical system, beside a condensed spot formed from a main beam used for recording/reproducing, the sub-beam having a light intensity lower than that of the main beam, and the sub-beam is used for detecting a servo signal such as a tracking error signal. In a typical optical system including the sub-beam, the laser light from the light source is divided by a diffraction grating etc., to form a single main beam and two or more sub-beams. In the recording layer of the optical disc, the sub-beams form a low-intensity spot in the vicinity of and separately from the spot formed by the main beam. Similarly, in the photodetector, the sub-beams are received by another light receiving part provided in the vicinity of the light receiving part that receives the main beam.
Usually, the power intensity of the sub-beams is set at around tenth or lower than the power intensity of the main beam. Accordingly, the ratio of intensity of the reflected light of the main beam reflected by the other recording layer to the intensity of the sub-beams is as large as twice or more times (namely, root of 10 times) relative to the interference to the signal light of the main beam, even in terms of the ratio of the electric field intensity. In addition, in the case where a deviation of the beam distribution within the section of the received light beam is to be detected, as in the case of a push-pull signal, the variation may occur only in the push-pull signal, if the interference occurs partially and unevenly irrespective of a small range of variation in the total amount of the received light. If the NA is as high as 0.85, such a problem may occur even if the system has a smaller cross-talk light.
A literature “International Symposium on Optical Memory 2004, Technical Digest, Th-I-06, “BD Pickup Head for Dual Layer Disc”, for example, describes a conventional technique for preventing the above cross-talk light. In the literature, part of the light in the beam section of the reflected light is diffracted by a diffraction element toward the direction away from the light reception, to thereby perform an equivalent shading effect and prevent the cross-talk light from reaching the photodetector. However, in this technique, since part of the signal light in addition to the cross-talk light is shaded and thus impossible to receive, there is a problem in that the signal itself is deteriorated.
It is an object of the present invention to provide an optical head and an optical disc drive, which are capable of reducing the ratio of the amount of received cross-talk light generated from the optical disc including two or more recording layers to the amount of the received signal light and thus having an improved data reproducing performance.
The present invention provides, in a first aspect thereof, an optical head that condenses a light from a light source onto a recording layer of an optical disc and includes a photodetector for receiving a reflected light from the recording layer as a signal light, the optical head including: a phase modulation plate disposed in an area in a vicinity of a center of an optical beam section of the signal light, the area passing therethrough a part of the reflected light overlapping the signal light on the photodetector among the reflected light reflected from a position in a vicinity of the recording layer as viewed along an optical axis, the phase modulation plate including a plurality of areas that provide different phase changes to a light passing through the phase modulation plate.
The present invention provides, in a second aspect thereof, an optical head that condenses a light from a light source onto a recording layer of an optical disc and includes a photodetector for receiving a reflected light from the recording layer as a signal light, wherein: condensed spots including a main beam and a plurality of sub-beams having an intensity lower than an intensity of the main beam are formed on the recording layer in the optical disc; and the optical head further includes a phase modulation plate disposed in an area in a vicinity of a center of an optical beam section of the signal light generated from the main beam, the area passing therethrough a part of the reflected light overlapping the signal light from the sub-beam on the photodetector among the reflected light reflected from a position in a vicinity of the recording layer as viewed along an optical axis, the phase modulation plate including a plurality of areas that provide different phase changes to a light passing through the phase modulation plate.
The present invention provides, in a third aspect thereof, an optical disc drive including an optical head that condenses a light from a light source onto a recording layer of an optical disc and includes: a photodetector for receiving a reflected light from the recording layer as a signal light; and a signal reproduction unit for reproducing data recorded on the recording layer from the signal light received by the photodetector, wherein: the optical head includes a phase modulation plate disposed in an area in a vicinity of a center of an optical beam section of the signal light, the area passing therethrough a part of the reflected light overlapping the signal light on the photodetector among the reflected light reflected from a position in a vicinity of the recording layer as viewed along an optical axis, the phase modulation plate providing different phase changes to a light passing through the phase modulation plate.
The present invention provides, in a fourth aspect thereof, an optical disc drive including: an optical head that condenses a light from a light source onto a recording layer of an optical disc and includes a photodetector for receiving a reflected light from a recording layer as a signal light; and a signal reproduction unit for reproducing data recorded on the recording layer from the signal light received by the photodetector, wherein: condensed spots including a main beam and a plurality of sub-beams having an intensity lower than an intensity of the main beam are formed on the recording layer in the optical disc; the optical head further includes a phase modulation plate disposed in an area in a vicinity of a center of an optical beam section of the signal light from the main beam, the area passing therethrough a part of the reflected light overlapping the signal light from the sub-beam on the photodetector among the reflected light reflected from a position in a vicinity of the recording layer as viewed along an optical axis, the phase modulation plate providing different phase changes to a light passing through the phase modulation plate.
In accordance with the optical head and optical disc drive of the present invention, since the two areas of the phase modulation plate provides different phase changes to the reflected light configuring the cross-talk light passing through the phase modulation, the transmitted lights passed by the respective areas cancel each other due to the phase change provided thereto. The ratio of part of the reflected light passed by the phase modulation plate among the signal light received by the photodetector to the total signal light is smaller than the ratio of the part of the cross-talk light passed by the phase modulation plate among the cross-talk light received by the photodetector to the total cross-talk. Accordingly, the ratio of the cross-talk light received by the light receiving part to the signal light is reduced, whereby the data reproduced from the signal light has an improved reliability.
Hereinafter, an optical head and an optical disc drive according to embodiments of the present invention will be described with reference to the accompanying drawings.
At this stage, a substantially central part of the cross-talk light 23 within the beam section thereof is received while overlapping the signal light 22. A phase modulation member 13 is provided in the central part of the convergent lens 11 through which the part of the cross-talk light 23 thus overlapping passes.
In either case of
The phase modulation member 13 generates an uneven phase change within the section of the transmitted light. For example, the phase modulation member 13 allows the light to pass therethrough without any phase change in a first area which occupies a half area of the phase modulation member 13, whereas the phase modulation member 13 allows the light to pass therethrough with a phase change of π in a second area which occupies the other half area thereof.
It is sufficient for the phase modulation member 13 to generate a phase difference between or among a plurality of areas. However, it is preferred that the cross-talk lights passed by the phase modulation member 13 cancel each other by 100% as a whole. For example, a phase difference of 0, π/2, 3π/2, and π may be allocated to respective four areas of the phase modulation member 13, as shown in
The phase modulation member 13 provides a phase change to the cross-talk light as well as the signal light passing therethrough. However, if a substantially whole of the signal light is received, the spot size on the photodetector 12 may be expanded to some extent, because the phase change provided by the phase modulation member 13 is only a phase change of a part of the beam section. Thus, if the output of the light receiving part of the photodetector 12 has a margin, the change of the received signal light is scarcely observed.
The phase difference in the phase modulation member 13 can be easily achieved as by attaching coated films having different thicknesses to a convergent lens, by diffracting whole the light beam while deviating the phase of the part of the diffraction element used therein.
The advantage of providing the phase modulation member 13 to control fluctuation of the received light signal depends on the received amount of the cross-talk light. This advantage is particularly effective in the case where the layer spacing between the recording layers is smaller than 40 micrometers, and where the NA of the optical system is smaller than 0.65.
In the optical head of
In addition to the above techniques, the optical head of the present invention may be combined with a configuration which adopts the well-known knife edge technique or three-beam technique, and may be applied to an optical head of an interchangeability type adapted to a plurality of standards of the optical disc.
If condensed spots including those of a main beam and two or more sub-beams are formed on an optical disc and a servo tracking error signal is to be detected from the sub-beams, as in the case of the three-beam technique, it is necessary to take into consideration the influence by the interference in the sub-beams to a higher degree than the in the main beam. This is because the sub-beams have an intensity smaller than the intensity of the main beam, and accordingly are liable to the influence of the interference by the cross-talk light.
If the light receiving part is divided into a plurality of areas, as shown in
It is a matter of course that if the number of sub-beams is increased up to three or more, such a configuration may be dealt with by providing the phase modulation members at the corresponding positions of the respective reflected light beams similarly to the above case. In addition, it is also possible to provide the phase modulation member only for a single sub-beam subjected to the marked influence by the cross-talk light, without providing the phase modulation member in the other sub-beam or sub-beams.
The configuration of the optical head of the present invention may be applied to any type of the optical head so long as the optical head includes an optical system including the phase modulation member at the position through which the cross-talk light interfering the signal light passes. In addition, the optical disc reproduced by the optical head of the present invention may be any optical disc including two or more recording layers, and any multilayer optical disc including three or more layers.
The signal from the optical head 31 is reproduced by a signal detection circuit 32, wherein the address signal is taken out simultaneously with the recorded information, to be delivered to the address decision circuit 33. The address decision circuit 33 judges the address position of the present optical head, to provide the present address position to the optical-head servo control circuit 34. The optical-head servo control circuit 34 performs servo control of the optical head 31 based on the difference between the present address position and the address position to be accessed, and locates the condensed spot at the desired address on the desired recording layer among the multiple recording layers.
Each recording layer of the optical disc includes an organic-material film, a dielectric film, a metallic reflection film etc. If the number of the recording layers is two, the movement of the condensed position of the optical spot between the layers can be performed with ease. If the number of recording layers is three or more, and a spherical aberration occurs depending on the condensed position, it is preferred to provide a compensation optical system in the optical system of the optical head 31.
As described heretofore, the optical head and optical disc drive of the present invention may have the following configurations. The phase modulation plate includes a plurality of groups of areas, the groups each providing different phases to the light passing through the phase modulation plate. The group of areas providing different phases allows the cross-talk signals to cancel each other, and the plurality of groups of the areas suppress the influence by the phase fluctuation inherently included in the signal light etc.
It is also a desirable embodiment wherein the groups each include at least two areas that provide different phases having a difference therebetween 180 degrees to the light passing through the phase modulation plate. In this case, the lights passed by both the areas have therebetween a phase difference of 180 degrees from each other, thereby achieving a complete cancellation.
The optical head and optical disc drive of the present invention are preferably used in the case where the optical disk includes two recording layers, and the optical head irradiates a laser light onto both the recording layers from one side of the optical disc. In this case, the amount of cross-talk light generated from both the recording layers is particularly reduced.
The present invention achieves a particularly remarkable advantage in the case where the layer spacing between the two recording layers is smaller than 40 micrometers, and the objective lens of the optical head has an NA of 0.65 or below. In this case, the ratio of the amount of the cross-talk signal to the amount of the signal light can be reduced down to 10% or less.
While the invention has been described with reference to preferred embodiments thereof, the optical head and optical disc drive of the present invention are not limited to the above embodiments, and modifications and alterations made from the above embodiments may fall within the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-301142 | Oct 2005 | JP | national |
| 2006-211542 | Aug 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/320308 | 10/11/2006 | WO | 00 | 4/16/2008 |
