OPTICAL READING DEVICE

The present invention relates to an optical scanning device for reproducing information from an optical disc and recording information on the disc.

An optical pickup device records and reproduces information, for example video and audio data, on and from storage media, e.g. discs. A disk has a structure comprising a substrate on which is formed a recording surface of the information. For example, the substrate can be plastic or glass. To read or writing information on a disc of high density, the diameter of the optical spot is to be very small. For this purpose, the numerical aperture of an objective lens is generally made long and is made of a light source shorter wavelength. However, in the case of using the light source of shorter wavelength, the tilt tolerance of the disgue relative to 1' optical axis is reduced. One may improve the tolerance of tilt of the disc, reduced, by reducing the thickness of the disk.

If is designated the angle of inclination of the disc by θ, the magnitude of a coma aberration coefficient W₃₁ is given by:

W31 d 2' n² (n² l-) sin0cosg ^> , (n² -sin² 0)² DD NA³

where d and n are the thickness and the refractive index of the disc, respectively. As shown in the above relationship, the coefficient of coma aberration is proportional to the cube of the numerical aperture (NA). Therefore, NA wherein the opening of the objective lens necessary for a conventional compact disc (CD) is 0.45 and the value for a digital video disc or a standard digital combined disc (DVD) is 0.6 (to account for the increased density of information), the DVD has a coefficient of coma aberration that is about 2.34 times that of the CD having the same thickness, for a given angle of inclination. Therefore, the tolerance of maximum inclination of the DVD is reduced to approximately half of that of the conventional CD. In order that the maximum tilt tolerance of the DVD is corresponding to the CD, to be reduced the thickness d of the DVD.

However, such a disc of reduced thickness associated with a light source shorter wavelength (high density), for example a DVD, cannot be used in a recording/reproducing apparatus such as a disk drive for the conventional CDS adopting a light source of longer wavelength, since a disk having a non-standard thickness is influenced by a spherical aberration to a degree corresponding to the difference in thickness of the disc relative to that of a normal disc. If the spherical aberration greatly increases, the spot formed on the disc cannot have the light intensity necessary for recording information, which prevents the accurate record of the information. Furthermore, during reproduction of the information, the signal-to-noise ratio (S/N) is too low for the accurate reproduction of the recorded information.

Therefore, an optical pickup device using a source of short wavelength light, for example 650 nm, which is compatible for discs having different thicknesses, such as a CD or DVD, is required.

For this purpose, studies are in progress for device capable of recording/reproducing information on one and the other of two types of disc having different thicknesses, with a same optical pick-up device employing a light source shorter wavelength. Lens devices using a combination of a hologram lens and a refractive lens have been proposed, for example, in the Japanese Patent The publication being No. Hei 7-98431.

Figures 1 and 2 illustrate the focus of a diffracted light of zero and first order on disks 3a and 3b having different thicknesses, respectively. On each figure, a hologram lens 1, design 11, a refractive lens and lens 2 are provided on the light path, in front of the discs 3a and 3b. The figure 11 diffracts a light beam 4 from a light source (not shown) and through the hologram lens 1, to separate the light through in a first order diffracted light 41 and zeroth order light 40, each ' is focused at a different point on the optical axis with a different intensity by the objective lens 2. The two different focal points are the foci on the thicker disk 3b and the disc thinner 3a, respectively, that enable operations information read/write on disks having different thicknesses.

However, when using such a lens system, the separation of the light into two beams (i.e. the order light 0 and the light of first order) by the hologram lens 1 decreases the efficiency of utilization of the light effectively used (reflected and partially diffracted twice, first order) to about 15%. Furthermore, during the read operation, since the information travel on one of the beams while the other beam is no information, the beam which does not carry any information may be detected as a noise. Furthermore, the manufacture of such a hologram lens requires a highly accurate technique for etching a fine pattern hologram, thereby increasing manufacturing costs.

Figure 3 is a schematic view of another optical pickup device known, as described in US patent No.5,281 797. The optical scanning device comprises a diaphragm the variable for varying the diameter of opening, so that data can be recorded on a disk with longer wavelength and on a disc having a shorter wavelength, but the disks having the same thickness, and the information can be reproduced from the disks. The the variable diaphragm is installed between the objective lens 2 and a collimating lens 5. The variable diaphragm controls a 4 the beam emitted by a light source 9 and transmitted through a beam splitter 6, by appropriately setting the surface of the transition region from the beam, i.e. the numerical aperture (NA). Is adjusted the diametrical opening of the diaphragm the variable as a function of the spot size required by the disc for use, to always pass the annular beam 4a of the central region but to stop or selectively transmitting the beam 4b of the peripheral region. On Figure 3, the mark 7 denotes a focusing lens 8 and the mark denotes a photodetector.

In the optical device with the above configuration, if the diaphragm variable element comprises a mechanical diaphragm, its resonance characteristics vary in dependence on structural of the effective aperture of the diaphragm. The installation of the diaphragm on a drive actuator of the objective lens becomes difficult in practice. To solve this problem, liquid crystals can be used to form the diaphragm.

However, this night much system miniaturization, night to the heat resistance and durability and increases manufacturing costs.

Alternatively, there may be a separate objective lens for each disk, so that an objective lens is used in specific for a specific disc. However, in this case, since a drive apparatus is required for changing the lenses, the configuration becomes complicated and the manufacturing cost increases in correspondence.

An object of the present invention is to provide an optical scanning device which is inexpensive and easy to manufacture.

Another object of the present invention is to provide an optical pickup device wherein the efficiency of utilization of light is enhanced and which can form small beam spots.

According to the present invention, the above objectives are achieved by an optical scanning device comprising: a light source; an objective lens provided along a light path from the light source, in front of a disc and having a predetermined effective diameter; a light-dividing element inserted in the path of light between the light control means and the light source; and a photodetector for detecting light divided by the light separating element and reflected by the disk.

In addition to the provisions claims, the invention further comprises other provisions which shall become apparent from the description that will follow.

The invention will be better understood with the aid of the complement of following description, that refers to the accompanying drawings in which:

figures 1 and 2 are diagrams of an optical pickup device having a conventional hologram lens, illustrating the conditions in which a light beam is focused on a thin disc and a thick disc, respectively;

figure 3 is a schematic view of another conventional optical pickup device;

figure 4 is a schematic view of an optical pickup device according to the present invention;

figure 5 is a plan view of a photodetector to eight segments used for the optical pickup device according to the present invention;

figures 6 to 8 and 9 to 11 are of plane views illustrating the distribution of light receiving of the photo-detector to eight segments, generated by the position of an objective lens with respect to a thin disk and a thick disc, respectively; and

figure 12 is a curve of a fireplace signal provided by the photodetector at eight segments shown in Figure 5.

It should be understood, however, that the drawings and the corresponding descriptive parts are given only by way of illustration of the object of the invention, which they form in no way a limitation.

In the present invention, among the beams of light incident on the photodetector, the beam of light about 1' central axis of the propagation path of the light, i.e. light of the intermediate region between the regions near and away from the axis (having many components of spherical aberration), is blocked or shielded, so that light having less components of spherical aberration reaches the photodetector, thereby stabilizing a fireplace signal. Therefore, a disc drive which is used compatibly for discs having different thicknesses, for example a compact disc of 1.2 mm and a digital video disc of 0.6 mm, is easily manufactured at a low cost.

Figure 4 is a schematic view of an optical pickup device according to the present invention, wherein is compares the states light focusing of a thin disc and of a thick disc.

On Figure 4, the indicia 300a and 300b denote a thin disc (such as a digital video disc of 0.6 mm) and a thick disc (for example a compact disc of 1.2 mm), respectively.

Usual An objective lens 200 is placed

in front of the digital video disc 300a or 300b of the compact disc. The objective lens 200 having a predetermined effective diameter focuses an incident light 400, from a light source 900, on the disc 300a or 300b, or receives the reflected light from the disk 300a or 300b. A beam splitter 700, to reflect light, from the light source 900 to the objective lens 200 and transmitting the reflected light from the disk 300a or 300b, is positioned between the objective lens 200 and the light source 900. A photodetector 820 as part of the present invention is placed behind a detector lens 800.

The photodetector 820 has the following structural characteristics.

The photodetector 820 is square in terms of its overall structure. A first detection region 821 divided into four parts is located at the center, and a second sensing region 822 divided into four parts is located around the first detection region 821.

The first detection region 821 comprises four square elements light detecting Al, Bl, Cl and Dl and the second detection region 822 light comprises four L-shaped elements light detecting A2, Β2, C2 and D2.

The first detection region 821 is large enough to encompass all incident light beams when the objective lens 200 is in a state of focus at the focal point with respect to the thin disc (digital video disc) 300a. The second detection region 822 receives light corresponding to the region extended by astigmatism when the objective lens 200 is not in focus position at the focal point. In particular, when the objective lens 200 is in-focus position with respect to the thick disc 300b, the second detection region 822 is the same size as a exterior square pattern tangentially to the region of light distribution. This is described below with reference to Figures 6 to 12, for a better understanding.

Figure 6 represents the distribution of light when the objective lens 200 is exactly in-focus position with respect to the thin disc 300a. The region of light distribution of the photodetector 820 820a is internally tangent to the first detection region 821.

Figure 7 represents the distribution of light when the objective lens 200 is in a position of remote focus, relative to the thin disc 300a. In this situation, as shown in Figure 7, light distribution region 820b is horizontally elongate, that is attached to, and is located on the horizontal members light receiving B2, Bl, Al, Cl, DI and D2.

Figure 8 represents the distribution of light when the objective lens 200 is in a position of near focus, relative to the thin disc 300a. In this situation, as shown in Figure 8, light distribution region 820c is vertically elongated, i.e. that it is on the vertical members light receiving A2, Al, Bl, DI, Cl and C2.

As shown in Figure 9, when the objective lens 200 is exactly in focus position at the focal point with respect to the thick disc 300b, light distribution region 820c is internally tangentée by the second detection region 822.

As shown in Figure 10, when the objective lens 200 is in a focus position remote from the thick disc 300b, light distribution region 820d is horizontally elongate, i.e. that it is on the horizontal members light receiving B2, Bl, Al, Cl, DI and D2.

As shown in Figure 11, when the objective lens 200 is in a position of near focus relative to the thick disc 300b, light distribution region 820th is vertically elongated, i.e. that it is on the vertical members light receiving A2, Al, Bl, Dl, Cl and C2.

As described above, in accordance with the present invention, only light near the axis, having a low spherical aberration, reaches the first detection region 821.

In the control of the optical pickup device according to the present invention, when reproducing or information is recorded from a thin disk or thereon (digital video disc) 300a, is used the signals generated by both the first and second detection regions 821 and 822. Therefore, when using the thin disc, the focus error signal is A1+A2 + C1 + C2-B1-B2-D1-D2. When is reproduced or recorded information from or on a thick disc, use the signal from the first detection region 821. Therefore, when using the thick disc, the focus error signal is A1+C1-B1-D1.

Figure 12 is a graph for comparing the signals from focus SI and S2, obtained from the signal detected by the first detection region 821 having a width of 60 um um and 90 respectively, with a fireplace signal S3 obtained from all the signals detected by the first and second detection regions 821 and 822 having a width of 160 μπΐ , when using the thick disc. Therefore, in the case of using the thick disc 300b, when using the first detection region 821, there can be obtained a fireplace signal SI more stable, is used relative to a case when both the first and second detection regions 821 and 822. On the other hand, when using the thick disc, light away from the axis, having a high spherical aberration, becomes largely distributed in the second detection region 822.

Therefore, the fireplace signal is increased and the symmetry for the focusing direction can be maintained.

As described above, with the optical pickup device according to the present invention, for reading information from two discs with different thicknesses, is used a light control film and a photodetector to eight segments, such that only light near the axis is received in the photodetector when the information is read from the disk and thick that the light and away from the axis is received in the photodetector when the information is read from the thin disc. The photodetector is divided into eight parts and each sensing region is subdivided into four parts.

Can be increased if necessary the number of the divided parts. Therefore, when using the thick disc, a signal is obtained corresponding to the region near the axis.

When using the thin disc, a signal is obtained relatively stable corresponding to the two regions, i.e. the regions near and away from the axis.

As described above, the optical reading device in accordance with the present invention employs a fixing element or scattering light which is simple and easy to manufacture, for example a light control film formed on a transparent member, or a locking groove or light scattering_e formed on the objective lens, while the optical reading device employs a conventional hologram lens complex and expensive. Furthermore, since light is used without being separated by a hologram lens, the optical reading device according to the present invention has an improved light use efficiency.

As spring of the foregoing, the present invention is not is limited remaining those of its embodiments and application that just described more explicit; it embraces on the contrary all the variants that may come to the mind of the technician in the art without departing from the framework or scope of the present invention.