ITMI961992A1 - OPTICAL READER - Google Patents

Description

Description of the industrial invention entitled: "OPTICAL READER"

Background of the invention

The present invention relates to an optical reader, and more particularly to an optical reader for reproducing and recording information from / on at least two types of discs having different thicknesses.

This optical player records and plays information such as video or audio data on / from recording media such as discs. A disc has such a structure that a surface recorded with information 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 disk, the optical dot diameter must be very small. For this purpose, the numerical aperture of a lens is generally made large and a light source having a smaller wavelength is used.

However, when using a shorter wavelength light source and a large numerical aperture (NA) lens, the possibility of tilting the disc relative to the optical axis is reduced. The reduced disc tilt capability can be increased by reducing the disc thickness.

Assuming that the angle of inclination of the disk is Θ, the amplitude of the coma aberration coefficient can be obtained from:

where d and n represent the thickness and refractive index of the disc, respectively. As can be understood from the above relationship, the coma aberration coefficient is proportional to the cube of the numerical aperture (NA). Hence, considering that the target NA required for conventional compact discs (CDs) is 0.45 and for a conventional digital video disc or digital versatile disc (DVD) it is 0.6 (to accommodate the higher information density ), DVD has a coma aberration coefficient of approximately 2.34 times that of CD having the same thickness for a given tilt angle. Thus, the maximum skew of DVD is reduced to about half that of conventional CD. In order to match the maximum skew of the DVD to that of the CD, the thickness d of the DVD may be reduced.

However, such a reduced thickness disc adopting a smaller wavelength (high density) light source, such as a DVD, cannot be used in a recording / reproducing apparatus such as a disc player for conventional CDS adopting a longer wavelength light source, because a disc having a non-standard thickness is affected 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 bright spot formed on the disc may not have the necessary light intensity to record information, which prevents the information from being recorded accurately. Again, during information playback, the signal-to-noise ratio (S / N) is too low to reproduce the recorded information exactly.

Hence, an optical reader is required adopting a light source having a small wavelength, for example 650 nm, which is compatible with discs having different thicknesses, such as a CD or a DVD.

For this purpose, a search is underway for devices that can play and record from / to at least two types of discs having different thicknesses with a single reading device that adopts a shorter wavelength light source. Lenses adopting a combination of a hologram lens and a refractive lens have been proposed, for example, in Japanese Patent Publication No. Hei 7-98431.

Figures 1 and 2 show the focusing of the zero-order and first-order diffracted light on disks 3a and 3b having different thicknesses, respectively. In each Figure, a hologram lens 1, provided with a profile 11, and a refractive lens 2, are provided along the path of the light in front of the discs 3a and 3b. The profile 11 diffracts a beam of light 4 coming from a light source (not shown) passing through the hologram lens 1, to thus separate the passing light into first order diffracted light 41 and zero order light 40, each of which is focused on a different point on the optical axis with a different intensity from the lens 2. The two different focal points are the appropriate focal points on the thicker disk 3b and the thinner disk 3a, respectively, and thus allow for reading / writing data with respect to disks having different thicknesses.

However, using such a lens system, the separation of light into two bands (i.e., zero-order and first-order light) by lens hologram 1 lowers the utilization efficiency of the light actually used (reflected and partially diffracted two times, first order) to about 15%. Again, during the reading operation, since the information is contained in only one of the two beams and the beam that does not carry information is likely to be detected as noise. In addition, the manufacture of such a hologram lens requires a high precision process used in etching a precise hologram design which increases manufacturing costs.

Figure 3 is a schematic diagram of another conventional optical reader as described in U.S. Pat. n. 5,281,797. This optical drive includes a variable diaphragm la to vary the aperture diameter so that data can be recorded on a longer wavelength disc as well as a shorter wavelength disc, but with discs having the same thickness, and information can be reproduced from them. The variable diaphragm la is installed between the objective 2 and a collimating lens 5. The variable diaphragm la controls a beam 4 emitted by a light source 9 and transmitted through a beam splitter 6, appropriately adjusting the area of the region through which the beam passes, i.e. the numerical aperture (NA). The diametrical aperture of the variable diaphragm la is adjusted according to the size of the point focused on the disk to be used and always allows the light ray 4a of the central region to pass, but it selectively passes or blocks the beam 4b of the peripheral region. In Figure 3, the reference number 3 indicates a disk, the reference number 7 indicates a focusing lens and the reference number 8 indicates a photodetector.

In the optical device having the above configuration, if the variable diaphragm is formed by means of a mechanical diaphragm, its structural resonance characteristics change according to the effective aperture of the diaphragm. Installing the diaphragm on an actuator to drive the lens becomes difficult in practice. To solve this problem, liquid crystals can be used to form the diaphragm. This, however, greatly prevents miniaturization of the system, deteriorates heat resistance and durability, and increases manufacturing costs.

Alternatively, a separate target can be provided for each disc, in order to use a specific target for a specific disc. In this case, however, since a control apparatus is required to change the lenses, the configuration becomes complex and consequently the manufacturing cost increases.

Summary of the invention

An object of the present invention is to provide an optical reader which is inexpensive and simple to manufacture.

Another object of the present invention is to provide an optical reader whose light utilization efficiency is improved and which can form points with reduced aberration.

To achieve this object, an optical reader is provided in accordance with the present invention comprising: a light source; an objective provided along a path of the light from the light source, facing the plane of a disc and having a predetermined effective diameter; a beam splitter provided between the lens and the light source; a photodetector for detecting the light reflected by the disk and divided by the beam splitter; and light control means provided in the light path, facing the photodetector for controlling the light of the intermediate region between regions near and far from the axis of a reflected light beam.

In the optical reader according to the present invention, a generally spherical lens or Fresnel lens can be used as the objective. The light control means can be provided separately in a separate element. Otherwise, the light control means can be provided in any optical element positioned along the light path directed to the photodetector or the photodetector itself.

The light control means may be provided as a type of a coating film for blocking, absorbing, diffusing or reflecting light. Furthermore, the light control means can be provided as an independent transparent element positioned along the light passage region or a light control groove for diffusing or reflecting light between regions near and far from the axis, positioned in any existing optical elements.

Furthermore, the light control film and the light control groove as the light control means may have an annular or peripherally polygonal shape (e.g. a square). Again, it is preferred that the light control groove be formed to be tilted at a predetermined angle or to be spherical, its bottom plane not being perpendicular to the light path, so that incident light can be reflected in a direction not parallel to the path of light.

In the optical reader according to the present invention, it is preferred that the photodetector have a detection region of a size corresponding to the reflected light of the region near the axis from the thick disk and the reflected light of the regions near and far from the axis from the thin disk. , so that only light near the axis is detected for the thick disc and both light near the axis and light away from the axis are detected for the thin disc.

Furthermore, it is preferred that the photodetector have a first detection region divided into multiple sub-regions and a second detection region divided into multiple sub-regions surrounding the first detection region. Under these conditions, the first detection region has a detection region of a size corresponding to the reflected light of the region near the axis from the thick disc and the reflected light of the regions near and far from the axis from the thin disc, so that only light near the axis is detected for the thick disc and both light near the axis and light away from the axis are detected for the thin disc. Furthermore, it is preferred that the second detection region receives the light outside the region near the axis, i.e. the light of the region far from the axis, depending on the distance between the disc and the lens.

Particularly, it is preferred that the first detection region of the photodetector and the second detection region surrounding the first detection region are both vertically and horizontally symmetrical in terms of overall structure. It is more preferred that the respective regions be divided into four parts to be so symmetrical.

Brief description of the drawings

The aforementioned purposes and advantages of the present invention will become clearer by describing in detail a preferred embodiment thereof with reference to the annexed drawings, in which:

Figures 1 and 2 are schematic diagrams of a conventional optical reader having a hologram lens showing the conditions under which a light beam is focused on a thin disc and a thick disc, respectively;

Figure 3 is a schematic diagram of another conventional optical reader;

Figure 4 is a schematic diagram of an optical reader according to the present invention;

Figure 5 is an extracted view showing the relationship between a photodetector and a light control film in the optical reader shown in Figure 4;

Figure 6 is a plan view of an eight-segment photodetector adopted in the optical reader according to the present invention;

Figures 7-9 are plan views showing the light receiving region formed on the eight segment photodetector, by the position of an objective relative to a thin disc and a thick disc, respectively; And

Figure 13 is a curve of a focus signal obtained from the eight-segment photodetector shown in Figure 6.

Detailed description of the invention

In the present invention, the light of the intermediate region between the regions near and far from the axis having more spherical aberration components is blocked, shielded or scattered so that the light having fewer spherical aberration components reaches the photodetector, thereby stabilizing a signal of focus. Hence, a disk drive which can be compatible with different types of discs having different thicknesses, such as a 1.2-inch compact disc and a 1.6mm digital video disc, is easily manufactured at low cost. The region near the axis represents the region around the central axis of the lens (defined optically as the optical axis) having a substantially negligible aberration, the region away from the axis represents the region which is furthest from the optical axis of the region near the axis, and the intermediate region is the region between the region near the axis and the region away from the axis.

Figure 4 is a schematic diagram of an optical reader according to the present invention, in which the light focusing conditions of a thin disc and a thick disc are compared. In Figure 4, the reference numbers 300a and 300b represent a thin disc (such as a 0.6mm digital video disc) and a thick disc (such as a 1.2mm compact), respectively.

A general objective 200 is positioned in front of the digital video disc 300a or compact disc 300b. Objective 200 having a predetermined effective diameter focuses an incident light 400 from a light source 900 onto the disc 300a or 300b, or receives the reflected light from the disc 300a or 300b. A quarter wavelength plate 500 is provided behind the lens 200. A beam splitter 700 is positioned between the quarter wavelength plate 500 and a collimating lens 600 adjacent to the light source 900.

A focusing lens 800, a light control element 810, according to a feature of the present invention, and a photodetector 820 are positioned along the path of the light reflected by the beam splitter 700. The photodetector 820 is electrically connected to a magnetic disk determiner. -optical 830. The disk determiner 830 is connected to a differential amplifier 841 and to a seeder 842 to obtain a photomagnetic signal.

In the optical reader according to the present invention having the above configuration, the light control element 810 is made of a transparent material and has on its surface a light control film 811 for absorbing, diffusing or reflecting the light of the intermediate region between the region near and far from the axis having many components of spherical aberration, between the light beams passing through it and traveling towards the photodetector 820.

In other words, as shown in Figure 5, the light control film 811 controls (e.g. blocks, absorbs, scatters, diffracts, refracts or reflects) the light of the intermediate region between the regions near and far from the axis having many components of spherical aberration. Hence, only the light beams of the regions near and far from the axis reach the photodetector 820. The light control film 811 for blocking the light of the intermediate region can have different shapes such as an annular ring or perimeter polygon) for example square or Pentagon.

Again, based on types, the light control film 811 may be provided as a coating film or a physical structure for blocking the path of the light. The light control film 811 which will be described below is to be understood in the broadest sense.

The photodetector 820 has the following structural characteristics. The 820 photodetector is square in terms of overall structure. A first detection region 821 divided into four parts is positioned in the center and a second detection region 822 divided into four parts is provided around the first detection region 821. The first detection region 821 comprises four square light detecting elements Al , B1, Cl and D1 and the second detection region 822 comprises four L-shaped light detecting elements A2, B2, C2 and D2.

The first detection region 821 is as large as an outer square pulled tangentially to the light distribution region produced by the light passing through the inner side of the light control film 821 when the lens 200 is in a focused condition with respect to the disk 300a digital video. In this condition, the second detection region 822 is large enough to enclose all the incident light beams beyond the light control film 821. This will be described with reference to Figures 7-12 for better understanding.

Figure 7 shows the light distribution when the lens 200 is in a focused condition relative to the digital video disc 300a. The light distribution region close to the axis passing through the inner side of the light control film 820 is internally tangent to the first detection region 821. The light passing through the outer side of the light control film 820 is strictly distributed only in the second detection region 822.

Figure 8 shows the light distribution when the lens 200 is in an out-of-focus condition with respect to the digital video disc 300a. In this condition, as shown in Figure 8, the light distribution lies horizontally, i.e., across the horizontal light-receiving elements B2, Bl, DI and D2.

Figure 9 shows the light distribution when the lens 200 is in an out-of-focus condition with respect to the digital video disc 300a. In this condition, as shown in Figure 9, the light distribution region 820c is vertically elongated, i.e. through the verticillae light-receiving elements A2, A1, Cl and C2.

Figure 10 shows the light distribution when the lens 200 is in a focused condition relative to the compact disk 300b. The light distribution region for light close to the axis passing through the inner side of the light control film 811 is internally tangent to the first detection region 821, as in the case of the digital video disc 300b. The light passing through the outer side of the light control film 811 is widely distributed in the second detection region 822 so that the second detection region 822 is internally tangent to the light distribution region for the light passing through the outer side of the light control film 811.

Figure 11 shows the distribution of light when the lens 200 is in an out-of-focus condition with respect to the conpact disk 300b. In this condition, as shown in Figure 11, the light distribution is stretched horizontally, i.e. through the horizontal light-receiving elements B2, Bl, DI and D2. In this condition, the light passing through the inner side of the light control film 811 is also distributed in the first detection region 821 and the light passing through the outer side of the light control film 811 is distributed to be stretched horizontally in the second detection region 822.

Figure 12 shows the light distribution when the lens 200 is in a near focus condition compared to the compact disk 300b. In this case, unlike Figure 11, the light distribution region is stretched vertically, i.e. through the vertical light-receiving elements A2, A1, Cl and C2. However, in this condition, the light passing through the inner side of the light control film 811 is also distributed in the first detection region 821.

As described above, according to the present invention, only the light passing through the inner side of the light control film 811, i.e. the near-axis light having small spherical aberration, reaches the first detection region 821.

In operating the optical player according to the present invention, when information is reproduced or recorded from / to a thin disc (digital video disc) 300a, the signals generated by both the first and second detection regions 821 and 822 are used. information is played back or recorded from / to a thick disc (compact disc) 300b, only a signal from the first detection region 821 is used.

Figure 13 is a curve for comparing a focus signal SI obtained from the signal generated only by the first detection region 821 in a condition where the light control film 811, according to a feature of the present invention, is used for the thick disk (compact disk) with a focus signal S2 obtained from all the signals generated by the first and second detection regions 821 and 822 in a condition where the light control film 811, according to a feature of the present invention, is not used for the thick disk (conpatto disk). The amplitude of the first detection region 821 for the photodetector used herein is selected at 90 μm, and that of the second detection region 822 surrounding the first detection region 821 is selected at 160 μm. Consequently, as shown in Figure 13, in case of use of the compact disk 300b, if the light control film 811 is adopted and the first detection region 821 is used, a more stable focus signal SI can be obtained, compared to the case where the first and second detection regions 821 and 822 are used. Again, when the compact disk is used, the light away from the axis which exhibits a high degree of spherical aberration is made to be widely distributed in the second region of detection 822. Thus, the focus signal S2 is increased and symmetry for the focusing direction can be maintained.

As described above, in accordance with the optical reader of the present invention, in order to read information from at least two types of discs having different thicknesses, i.e. a compact disc and a digital video disc, a light control film and a photodetector are adopted. eight segments so that only light near the axis is received in the photodetector when the information is read from the compact disc and light near and far from the axis is received in the photodetector when the information is read from the digital video disc. Thus, when the thick disk is used, a signal corresponding to the region close to the axis is obtained. When using the thin disk, a relatively stable signal is obtained corresponding to both regions, i.e. the one near and far from the axis.

As described above, the optical reader according to the present invention, compared to conventional optical readers, adopts light blocking or diffusion means which are simple and easy to manufacture, for example a light control film formed on a transparent element or a light blocking or diffusion groove formed on a lens; while conventional optical readers adopt a complex and expensive hologram lens. Furthermore, since the light is used without being separated by a hologram lens, the optical reader according to the present invention has an improved light utilization efficiency. Finally, since a signal is obtained which can discriminate the disc type, a separate element is not required to discriminate the disc type.

Claims (17)

CLAIMS 1. An optical reader comprising: a light source; - an objective provided along the path of the light coming from said light source, facing the plane of a disc and having a predetermined effective diameter; a beam splitter provided between said objective and said light source; a photodetector for detecting the light divided by said beam splitter and reflected by the disk; light control means provided along the light path, facing said photodetector for controlling the light of the intermediate region between regions near and far from the axis of an incident light beam. 2. Optical reader according to claim 1, wherein said light control means is provided in a separate transparent element. 3. An optical reader according to claim 1, wherein said light control means is provided by a coating film which can control the light emitted by said light source. 4. An optical reader according to claim 2, wherein said light control means is provided by a coating film which can control the light emitted by said light source. 5. Optical reader according to claim 3, wherein said photodetector is divided into a first detection region divided into multiple parts and a second detection region divided into multiple parts. 6. Optical reader according to claim 4, wherein said photodetector is divided into a first detection region divided into multiple parts and a second detection region divided into multiple parts. 7. Optical reader according to claim 5, wherein said first detection region has the size corresponding to the light near the axis reflected by a thick disk, and to the light near and far from the axis reflected by a thin disk. An optical reader according to claim 6, wherein said first detection region has the size corresponding to the light near the axis reflected by a thick disk and the light near and far from the axis reflected by a thin disk. 9. Optical reader according to claim 7, wherein said first and second detection regions of said photodetector are symmetrically divided into four parts and form a square in terms of overall structure. 10. Optical reader according to claim 8, wherein said first and second detection regions of said photodetector are symmetrically divided into four parts and form a square in terms of overall structure. 11. An optical reader according to claim 1, wherein said photodetector has the dimension corresponding to the light near the axis reflected by a thick disk and to the light near and far from the axis reflected by a thin disk; and wherein said photodetector is divided into a first detection region divided into multiple parts and a second detection region divided into multiple parts. 12. An optical reader according to claim 2, wherein said photodetector has the dimension corresponding to the light near the axis reflected by a thick disk and to the light near and far from the axis reflected by a thin disk; and wherein said photodetector is divided into a first region divided into multiple parts and a second detection region divided into multiple parts. 13. An optical reader according to claim 11, wherein said first detection region has the dimension corresponding to the light near the axis reflected by a thick disk and to the light near and far from the axis reflected by a thin disk. An optical reader according to claim 12, wherein said first detection region has the size corresponding to the light near the axis reflected by a thick disk and the light near and far from the axis reflected by a thin disk. 15. Optical reader according to claim 13, wherein said first and second detection regions of said photodetector are symmetrically divided into four parts and form a square in terms of overall structure. 16. Optical reader according to claim 14, wherein said first and second detection regions of said photodetector are symmetrically divided into four parts and form a square in terms of overall structure. 17. Optical reader as claimed in claim 1, wherein said light control means are provided on said photodetector.