A lens device which can be used as an objective lens in an optical pickup apparatus includes an objective lens provided along a light path facing a disc and having a predetermined effective diameter, and light controlling means provided along the light path for controlling the light in an intermediate region between near and far axes of an incident light beam, thus providing a simplified and inexpensive device for using discs of differing thickness in a single disc drive, by reducing the spherical aberration effect.
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0. 27. A lens for use with optical memory disks of at least two types, each type being distinguished from another by having information bearing levels at different locations along axes of said optical memory disks, comprising:
a near axial region which includes a center of a light path;
an intermediate axial region being located radially outward from said near axial region; and
a far axial region located radially outward from said intermediate region,
wherein said near region focuses light in said light path on the information bearing level regardless of which of said at least two types of optical memory disks.
1. A lens device comprising:
a lens focusing light into a focal zone and having a predetermined effective diameter; and
light controlling means provided in a light path of said lens for preventing light in an intermediate axial region of said light path from reaching said focal zone, said intermediate axial region being located between near axial region which includes a center of said light path and a far axial region located radially outward from said intermediate region, said light controlling means permitting light in said near and far regions of said light path to reach said focal zone without imposing a relative phase change between light in said near and far regions.
0. 30. An optical pick-up device for use with optical memory disks of at least two types, each type being distinguished from another by having information bearing levels at different locations along axes of said optical memory disks, comprising:
a light source;
an objective lens;
a photodetector which detects light transmitted through said objective lens and focused on said photodetector after being reflected by a disk;
wherein said lens includes
a near axial region which includes a center of a light path;
an intermediate axial region being located radially outward from said near axial region; and
a far axial region located radially outward from said intermediate region,
wherein said near region focuses light in said light path on the information bearing level regardless of which of said at least two types of optical memory disks.
0. 37. An optical pick-up device for use with optical memory disks of at least two types, each type being distinguished from another by having information bearing levels at different locations along axes of the optical memory disks, comprising:
a light source;
an objective lens;
a photodetector which detects light transmitted through said objective lens and focused said photodetector after being reflected by a disk;
light controller provided in a light path of said lens which controls light in said light path before reaching said photodetector, said light controller includes a near axial region which includes a center of said light path, an intermediate axial region being located between near axial region and a far axial region located radially outward from said intermediate region, said light controller permitting light in said near region of said light path to focus on the information bearing level regardless of which of said at least two types of optical memory disks such that light in said near axial region reaches said photodetector.
0. 32. A lens device for use with optical memory disks of at least two types, each type being distinguished from another by having information bearing levels at different locations along axes of said optical memory disks, comprising:
a lens focusing light into said information bearing levels and having a predetermined effective diameter; and
a light controller provided in a light path of said lens which controls light in said light path before reaching said information bearing levels, said light controller includes a near axial region which includes a center of said light path, an intermediate axial region being located between the near axial region and a far axial region located radially outward from said intermediate region, said light controller permitting light in said near region of said light path to focus on the information bearing level regardless of which of said at least two types of optical memory disks, said light controller permitting light in said near and far regions of said light path to reach said focal zone without imposing a relative phase change between light in said near and far regions.
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The present invention relates to a lens device and method of making same, a method of obtaining a stable focus servo signal, an optical pickup adopting the same, a method of discriminating discs having different thicknesses and a method of reproducing/recording information from/onto the discs.
An optical pickup records and reproduces information such as video or audio data onto/from recording media, e.g., discs (or disks). A disc has a structure that an information-recorded surface is formed on a substrate. For example, the substrate can be made of plastic or glass. In order to read or write information from a high-density disc, the diameter of the optical spot must be very small. To this end, the numerical aperture of an objective lens is generally made large and a light source having a shorter wavelength is used. However, in case of using the shorter wavelength light source, a tilt allowance of the disc with respect to optical axis is reduced. The thus-reduced disc tilt allowance can be increased by reducing the thickness of the disc.
Assuming that the tilt angle of the disc is θ, the magnitude of a coma aberration coefficient W31 can be obtained from:
where d and n represent the thickness and refractive index of the disc, respectively. As understood from the above relationship, the coma aberration coefficient is proportional to the cube of the numerical aperture (NA). Therefore, considering that the NA of the objective lens required for a conventional compact disc (CD) is 0.45 and that for a conventional digital video disc or digital versatile disc (DVD) is 0.6 (to accommodate the higher information density), the DVD has a coma aberration coefficient of about 2.34 times that of the CD having the same thickness for a given tilt angle. Thus, the maximum tilt allowance of the DVD is reduced to about half that of the conventional CD. In order to conform the maximum tilt allowance of the DVD to that of the CD, the thickness d of the DVD could be reduced.
However, such a thickness-reduced disc adopting a shorter wavelength (high density) light source, e.g., a DVD, cannot be used in a recording/reproducing apparatus such as a disc drive for the conventional CDs adopting a longer wavelength light source because a disc having an non-standard thickness is influenced by a spherical aberration to a degree corresponding to the difference in disc thickness from that of a normal disc. If the spherical aberration is extremely increased, the spot formed on the disc cannot have the light intensity needed for recording information, which prevents the information from being recorded precisely. Also, during reproduction of the information, the signal-to-noise (S/N) ratio is too low to reproduce the recorded information exactly.
Therefore, an optical pickup adopting a light source having a short wavelength, e.g., 650 nm, which is compatible for discs having different thicknesses, such as a CD or a DVD, is necessary.
For this purpose, research into apparatuses capable of recording/reproducing information on either of two disc types having different thicknesses with a single optical pickup device and adopting a shorter wavelength light source is under progress. Lens devices adopting a combination of a hologram lens and a refractive lens have been proposed in, for example, Japanese Patent Laid-Open Publication No. Hei 7-98431.
However, in using such a lens system, the separation of the light into two beams (i.e., the zero order and first order light) by the hologram lens 1 lowers the utilizing efficiency of the actually used (reflected and partially twice diffracted, 1st order) light to about 15%. Also, during the read operation, since the information is riding on one of the beams while the other beam is carrying no information, the beam that is carrying no information is likely to be detected as noise. Moreover, the fabrication of such a hologram lens requires a high-precision process used in etching a fine hologram pattern, which increases manufacturing costs.
In the optical device having the above configuration, if the variable diaphragm is formed by a mechanical diaphragm, its structural resonance characteristics change depending on the effective aperture of the diaphragm. The installation of the diaphragm onto an actuator for driving the objective lens becomes difficult in practice. To solve this problem, liquid crystals may be used for forming the diaphragm. This, however, greatly impedes the miniaturization of the system, deteriorates heat-resistance and endurance and increases manufacturing costs.
Another approach is disclosed in U.S. Pat. No. 5,496,995. As disclosed, a phase plate in placed in a light path of an objective lens. The phase plate creates first and second light sources of different phases such that the amplitudes of the lateral sides of a main lobe of an image of the first light source are cancelled by the amplitude of the main lobe of an image of the second light source by superimposition. In one embodiment, annular opaque rings separate grooves of different depths, the grooves providing the phase difference. A problem inherent to this approach is the need to carefully control the groove depth and light amplitudes, for example, to create the proper phase change and lobe cancellation.
Alternatively, a separate objective lens for each disc may be provided so that a specific objective lens is used for a specific disc. In this case, however, since a driving apparatus is needed for changing lenses, the configuration becomes complex and the manufacturing cost increases accordingly.
It is an object of the present invention to provide a lens device which is inexpensive and easily fabricated, a method of obtaining a stable focus servo signal, an optical pickup adopting the same, a method of discriminating discs having different thicknesses and a method for reproducing/recording information from/onto the discs.
It is another object of the present invention to provide an objective lens whose light utilizing efficiency is enhanced and which can form aberration-reduced spots, a method of obtaining a stable focus servo signal, an optical pickup adopting the same and a method for reproducing/recording information from/onto the discs.
To accomplish the above objects, there is provided lens device including a lens focussing light into a focal zone and having a predetermined effective diameter; and light controlling means provided in a light path of the lens for preventing light in an intermediate axial region of the light path from reaching the focal zone, the intermediate axial region being located between a near axial region which includes a center of the light path and a far axial region located radially outward from the intermediate region, the light controlling means permitting light in the near and far regions of the light path to reach the focal zone.
Also, according to another aspect of the present invention, there is provided an optical pickup device comprising:
Also, according to still another aspect of the present invention, there is provided a method for reproducing information from at least two discs having different thicknesses, comprising the steps of:
Also, there is provided a method for recording information on at least two discs having different thicknesses, comprising the steps of:
Further, there is provided a method for discriminating discs having different thicknesses, comprising the steps of:
Additionally, there is provided a method for manufacturing a lens comprising the steps of:
Also, there is provided a lens mold for forming a lens, the lens configured to focus light into a focal zone, the lens mold comprising:
The above objects and advantages of the present invention will become more apparent from the following detailed description preferred embodiments thereof with reference to the attached drawings in which:
In the present invention, the light in an intermediate region around an axis in the center of a light travelling path is blocked or shielded. The intermediate region is located between a region near the axis (“near axial region”) and a region farther from the axis (“far axial region”). Blocking the light in the intermediate region permits the light from the near and far axes regions to form a small light spot while minimizing side lobes around the light spot formed in a focal zone of the lens by suppressing interference of light otherwise present in the intermediate region.
Here, the near axis region represents the region around the central axis of the lens (i.e., the optical axis) having a substantially negligible aberration and focussing on a region adjacent to the paraxial focal point. The far axis region represents the region which is relatively farther from the optical axis than the near axis region and forms a focus region adjacent to the marginal focus. The intermediate region is the region between the near axis region and the far axis region.
Alternately, a near axis region and a far axis region can be defined by the optical aberration amount in a thick disc. An objective lens must have very small amount of optical aberration (e.g., spherical aberration, coma, distortion, etc.). Generally, an objective lens should have average aberration below around 0.04λ (where λ denotes the wavelength of light transmitted to the lens) in order to use in an optical pickup device. An objective lens having optical aberration greater than 0.07λ is considered as unacceptable for use in an optical pickup device. As the thickness of the disc increases, the optical aberration increases. Thus, if the objective lens having optical aberration below 0.04λ is used for a pre-defined or thin disc (e.g., DVD), it produces a large amount of optical aberration (mainly spherical aberration) for a thicker disc (e.g., CD).
Furthermore, the unwanted peripheral light (B) shown in
To this end, in the intermediate region between the near axis and the far axis regions along the incident light path, there is provided light controlling means of an annular shape or a polygonal shape such as a square shape for blocking or scattering light. This invention utilizes the fact that the light of the far axis region does not affect the central light portion of the light spot but the light of the intermediate region between the near axis and the far axis does.
As described above, the light spot formed on a thick disc is larger than that formed on a thin disc, which is due to the spherical aberration. Also, since the light incident onto a far axis region, i.e., a region relatively far from the optical axis, is focused onto an area different (surrounding) from the optical axis and is scattered, the light of the far axis region does not affect the focusing of the light spot of the central part (A). However, as described above, since the light present between the near axis and the far axis interferes with the focusing of the light of the near axis, the amount of the peripheral light (B) of the focused light becomes greater. In other words, the light in the intermediate region between the near axis region and the far axis region experiences interference when the present invention is not employed, so that peripheral light beams (B) are generated around the central light beam (A), as shown in FIG. 5. Such peripheral light beams generally have about 6˜7% intensity of the central light beam, thereby increasing jitter during light detection and thus making accurate data recording and reproduction difficult.
Under the above conditions, graphs (c) and (d) are curves showing the change in light spot sizes in case of adopting a 0.6 mm disc and graphs (a) and (b) are ones in case of adopting a 1.2 mm disc. Here, graphs (b) and (c) show the spot state present when the present invention is adopted.
It is understood that the difference in spot size at central portion “A” of
Therefore, as described above, according to the present invention, the light passing through the intermediate region between the near axis and the far axis regions is controlled. For this purpose, there is provided along the light path a light controlling means for controlling (e.g., blocking, scattering, diffracting, absorbing or refracting) the light in the intermediate region, thereby suppressing an increase in the size of the peripheral light of the light spot and reducing the spherical aberration which would otherwise occur.
In
A general objective lens 200 is positioned in front of disc 300a or 300b. The objective lens 200 having a predetermined effective diameter focuses an incident light 400 from a light source 900 and receives the light reflected from disc 300a or 300b. As shown in
A collimating lens 500 and a beam splitter 600 are provided between light controlling member 100 and light source 900, as shown in
In the optical pickup device having the aforementioned configuration according to the present invention, the light controlling film 101 suppresses among the incident light beams 400, the light beam 402 of the intermediate region passing through the region between the near axis and the far axis regions, thereby transmitting only the light beams 401 and 403 passing through the near and far axes regions, as shown in FIG. 9. For example, a light controlling film 101 made of Chromium (Cr) would block the light beam 402 from passing through the light controlling member 100. Moreover, the light beam 402 can be scattered, reflected, diffracted or refracted depending on the surface roughness of the light controlling film 101.
The light controlling film 101 having the above-described function is directly coated on one surface of objective lens 200, as shown in FIG. 10. As shown in
Like the aforementioned light controlling film 101, the light controlling groove or wedge-shaped rib 102 is provided in the light region between the near axis and the far axis and functions to redirect (e.g., reflect, refract or scatter) the incident light in a direction irrelevant to the light focusing or suppress (e.g., block) the incident light.
The objective lens 200′ can be manufactured by a general high-pressure injection molding method (not shown) or a compression molding method, as shown in
The lower mold 1002a has a pattern having one or multiple grooves 103a formed in correspondence with the light controlling rib 102 for dispersing the light in the intermediate region, as shown in
In
The light controlling groove 102 is preferably formed for the bottom surface of the objective lens 200′ to be oriented by a predetermined angle θ with respect to a perpendicular of the optical axis, as shown in FIG. 14B. The light of the intermediate region, reflected from light controlling groove 102, is preferably scattered or reflected in a direction not parallel to the optical axis.
Light controlling groove 102′ can be formed of a polygonal shape such as a square shape. Moreover, the objective lens can be modified to have more than one light controlling groove to control the incident light. It is also possible to use any of these surface irregularities (e.g., groove, rib, toothed, rough and jagged) on a separate transparent member such as the light controlling members 100.
In the above embodiments, a convex lens was used as the objective lens 200 or 200′, which might be replaced by a planar lens using a diffraction theory, such as a hologram lens or a Fresnel lens. Specifically, when the lens is provided with light controlling means, an annular or square light controlling groove 102″ is formed in a plane lens, as shown in
A light controlling film 101 shown in
It should be noted that the lens device structure described above is not limited to an objective lens used in an optical pickup device.
In the light spot formed under the above conditions, as the result of the measurement, the diameter of the light spot at a point of 1/e2 (˜13%) of the central light intensity was 1.3 μm. Compared to the device shown in
As described above, according to the present invention, a light spot can be formed on a disc at an optimal state. As shown in
Hereinbelow, the characteristics of the photodetector 800 in the optical pickup device according to the present invention will be described in detail.
As shown in
First, referring to
Referring to
As described above, in order to read information from discs having different thicknesses, the optical pickup device according to the present invention adopts a photodetector 800 devised so as to receive only the light of the near axis region in reading information from a thick disc and receive the light of the near and far axes regions in reading information from a thin disc. Therefore, when a thick disc is used, a signal corresponding to the light of the near axis region is obtained. When a thin disc is used, a relatively higher intensity signal, corresponding to the light of the near and far axes regions, is obtained.
A focus error signal obtained by using the octahedron photodetector 810 is shown in
The first detection region 811 has dimensions such that the size of the first region 811 should be optimized to receive the light from the near axis region without loss when information is read from a thick disc, and not to receive the light from the far axis region. Additionally, the first and second detection regions 811 and 812 have dimensions such that the light beams of the near axis and far axis regions are all received when information is read from a thin disc, as shown in FIG. 24. When information is read from a thick disc, the light of the far axis region impinges on the second light receiving region 812, as shown in FIG. 27.
In the photodetector 810 having the aforementioned structure, the entire signal, i.e., that from both the first and second light receiving regions 811 and 812, is used in reading information from a thin disc, and only the signal from the first light receiving region 811 is used in reading information from a thick disc.
As understood from the above, when information is read from a thick disc, the focus error signal components are obtained by using only the light of the near axis region, thereby obtaining a stable focus error signal as shown in FIG. 30.
As described above, in the focus controlling method of the objective lens device and optical pickup device adopting the same according to the present invention which has a size reducing effect of the light spot, i.e., the light amount of the portion “B” of
Also, the magnitudes of the detected focus error signals are different depending on the disc thickness. In other words, as shown in
The operation of discriminating the disc type will now be described in detail with reference to the flowchart of FIG. 32.
If a thin or thick disc is inserted (step 100), focus current (which controls the position of the objective lens relative to the disc) is increased or decreased to discriminate the range of an objective lens, i.e., the type of a disc, as shown in FIG. 33. The objective lens is moved up and down m times (m=1, 2, 3, . . . ) within its range of focus adjustment, thereby obtaining a sum signal from the photodetector (adding together all signals from each of the eight quadrants) and a focus error signal (Sf) (step 101). Since a quadrant photodetector is used, the focus error signal is obtained by a conventional astigmatic method such as disclosed in U.S. Pat. No. 4,695,158 to Kotaka et al, for example. Being conventional, an explanation thereof will not be belabored. Experimentally it has been shown that the amplitude of the focus error signal for a thin disc reproduction is four times that for a thick disc reproduction, that the light intensity is enough for compatibility with both disc types and that a focus error signal stabilization is realized.
The amount of spherical aberration is reduced by the above-described method to reproduce a signal recorded onto a disc. However, the spherical aberration is larger than that of the optical pickup for the conventional compact disc player, thereby resulting in the deterioration of a reproduction signal. Therefore, it is preferable that a digital waveform equalizer is used, such as shown in
fo(t)=fi(t+τ)−K[fi(t)+fi(t+2τ)]
where τ is a predetermined delay time, and K is a predetermined amplitude divider, as shown in
Once the focus error signal Sf and the sum signal are obtained (step 101), it is determined whether the focus error signal Sf is greater than a first reference signal for a thin disc (step 102). At this time, the sum signal may be also compared with the first reference signal in accordance with the design conditions.
As shown in
As shown in
If the focus error signal Sf or the sum signal is smaller than a second reference signal, an error signal is generated (step 123). The focus error signal and the sum signal can be used to discriminate the disc type clearly and this method using both signals reduces the discrimination error.
As described above, the lens device according to the present invention has various advantages as follows.
The lens device according to the present invention adopts a light blocking or scattering means which is simple and easy to fabricate, instead of a complex and expensive hologram lens. Also, since the light can be used without being separated by a hologram lens, the lens device has higher light utilizing efficiency than that of the conventional device. In addition, since a very small beam spot is formed, the performance of recording and reproducing information can be enhanced. Since the lens device with a light blocking, refracting, diffracting or scattering means has a single objective lens, it is very simple to assemble and adjust the optical pickup adopting the lens device. Also, since a signal which can discriminate the disc type is always obtained regardless of the thickness of the discs, additional means is not required for discriminating the disc type. In contrast, the conventional device using hologram has to adopt additional means to discriminate some signals because the device generates multiple signals. Among the multiple signals, one is used for thin discs and another is used for thick discs.
While the invention has been particularly shown and described with reference to a preferred embodiments thereof, it will be understood by those skilled in the art that various other changes in form and details may be made therein without departing from the spirit and scope of the invention. For instance, the relative position of the discs in the light path can be altered, thereby changing the spot patterns and consequently the details of the various methods using the electrically converted spot patterns.
Cho, Kun-ho, Lee, Chul-Woo, Chung, Chong-Sam, Shin, Dong-Ho, Seong, Pyong-Yong, Yoo, Jang-Hoon, Lee, Yong-Hoon, Rim, Kyung-hwa
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