GB2306045A

GB2306045A - Optical pickup device

Info

Publication number: GB2306045A
Application number: GB9620677A
Authority: GB
Inventors: Kun-Ho Cho; Eung-Ho Kim; Jang-Hoon Yoo; Kyung-Hwa Kim
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1995-10-04
Filing date: 1996-10-04
Publication date: 1997-04-23
Anticipated expiration: 2016-10-04
Also published as: FR2739717A1; DE19640837A1; FR2739717B1; GB9620677D0; KR970023055A; GB2306045B; DE19640837B4; KR100200858B1; NL1004079C2; ITMI961992A1; JPH09120572A; NL1004079A1; IT1285223B1; JP3872145B2

Description

1 OPTICAL PICKUP DEVICE 2306045

Backaround to the Invention

The present invention relates to an optical pickup device for reproducing and/or recording information from/onto an optical disc.

An optical pickup records and reproduces information such as video or audio data onto/from recording media, e.g., discs. The disc structure includes an information-carrying surface 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, when using the shorter wavelength light source, the tilt allowance of the disc with respect to optical axis is reduced. The reduced disc tilt allowance can be increased by reducing the thickness of the disc.

Assuming that the tilt angle of the disc is 0, the magnitude of a coma aberration coefficient W31 can be obtained from:

W31 _ n2(n2-1)sinf)cosO NA3 2 2 (n 2 -sin 20) 2 where d and n represent the thickness and refractive index of the disc, respectively. As will be 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), a WD has a coma aberration coefficient of about 2.34 times that of a CD having the same thickness for a given tilt angle. Thus, the maximum 2 tilt allowance of the WD is reduced to about half that of the conventional CD. In order to conform the maximum tilt allowance of the WD to that of the CD, the thickness d of the WD could be reduced.

However, such a thickness-reduced disc using 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 conventional CDS using a longer wavelength light source because a disc having an non-standard thickness is influenced by spherical aberration to an extent corresponding to the difference in disc thickness from that of a normal disc. If the spherical aberration is greatly 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 signalto-noise (SIN) ratio is too low to reproduce the recorded information accurately.

Therefore, an optical pickup using a light source having a short wavelength, e.g., 650nm, which is compatible for discs having different thicknesses, such as a CD or a DVD, is necessary.

For this purpose, research into apparatus capable of recording/ reproducing information on either of two disc types having different thicknesses with a single optical pickup device and using a shorter wavelength light source is under progress. Lens devices using a combination of hologram lenses and refractive lenses have been proposed in, for example, Japanese Patent Publication No. Hei 798431.

FIGS. 1 and 2 show the focusing of zero-order and firstorder-diffracted light onto discs 3a and 3b having different thicknesses. In each figure, a hologram lens 1, provided with a pattern 11, and a refractive objective lens 2 are provided along the light path in f ront of discs 3a 3 and 3b. The pattern 11 dif f racts a light beam 4 f rom a light source (not shown) passing through hologram lens 1, to separate the transmitted light into first-orderdiffracted light 41 and zero-order light 40, each of which is focused onto a different point on the optical axis with a different intensity by the objective lens 2. The two different focal points are the appropriate focus points on the thicker disc 3b and the thinner disc 3a and thus enable data read/write operations for discs of different thicknesses.

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 used (reflected and partially twice diffracted, lst order) light to about 15. Also, during read operations, 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.

FIG. 3 is a schematic diagram of another conventional optical pickup device as disclosed in U.S. Patent No. 5,281,797. This optical pick-up device includes a variable diaphragm la for varying the aperture diameter, so that data can be recorded onto a longer wavelength disc as well as a shorter wavelength disc, but with the discs having the same thickness, and information can be reproduced from them. The variable diaphragm la is positioned between the objective lens 2 and a collimating lens 5. The variable diaphragm la controls a beam 4 emitted from a light source 9 and transmitted through a beam splitter 6, by appropriately adjusting the area of the beam passing region, i.e., the numerical aperture (NA). The diametric aperture of the variable diaphragm la is adjusted in accordance with the spot size required by the disc to be 4 used and always passes the annular beam 4a of the central region but selectively passes or blocks the beam 4b of a peripheral region. In FIG. 3, reference numeral 7 denotes a focusing lens and reference numeral 8 denotes a 5 photodetector.

In an 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.

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.

It is an object of the present invention to provide an 25 optical pickup device which is inexpensive and easily fabricated.

It is another object of the present invention to provide an optical pickup device whose light utilizing efficiency is enhanced and which can form aberration-reduced spots.

Summary of the Invention Accordingly, the present invention provides an optical pickup device comprising: 35 a light source; an objective lens having a predetermined effective diameter and adapted to focus light from the light source towards the plane of a disc; a photodetector for detecting light reflected from the disc back through the objective lens; and light controlling means for interrupting the transmission of light of an intermediate region between near- and far axis regions.

The objective lens may be a spherical lens or a Fresnel lens.

Preferably, the device further includes a light splitter 10 for transmitting light from the light source to the objective lens and reflecting reflected light from the objective lens to the photodetector.

The light controlling means may be a separate transparent member and/or it may comprise a coating film for controlling the light emitted from the light source. The light controlling means may be provided in any optical element or elements positioned along the light path.

The light controlling means may be a coating film for blocking, absorbing, scattering or reflecting the light. Alternatively, the light controlling means may be an independent transparent member positioned along the light passing region or a light controlling groove for scattering or reflecting the light between near- and far axis regions positioned in any optical element.

The light controlling film and/or groove may have an annular or polygonal (e.g., a square) shape. It is preferred that the light controlling groove be formed at a predetermined angle, such that the bottom plane of the groove is not perpendicular to the light path, so that the incident light is reflected in a direction not parallel to the light path.

Preferably, the photodetector includes a first detection region and a second detection region. The first and second detection regions may be divided into multiple parts. For example, the first and second detection regions of the 6 photodetector may be symmetrically divided into four parts and form a square.

Preferably, the first detection region is an inner region 5 and the second detection region is a peripheral region.

Preferably, the first detection region is of a size corresponding to nearaxis light reflected from a disc of 1. 2mm thickness or more and corresponding to near- and f ar- axis light ref lected f rom a disc of 0. 6mm thickness or less.

Pf npgcription of the Drawings The above objects and advantages of the present invention will become more apparent from the following description of a preferred embodiment with reference to the accompanying drawings in which:

FIGS. 1 and 2 are schematic diagrams of a conventional optical pickup device having a hologram lens, showing a light beam focused onto a thin disc and a thick disc respectively; FIG. 3 is a schematic diagram of another conventional optical pickup device; FIG. 4 is a schematic diagram of an optical pickup according to the present invention; FIG. 5 shows the relationship between a photodetector and a light controlling film in the optical pickup device shown in FIG. 4; FIG. 6 is a plan view of an eight-segment photodetector used in the optical pickup device; FIGS. 7-9 are plan views showing the light receiving region formed on the eight-segment photodetector, by an objective lens position relative to a thin disc and a thick disc; and FIG. 13 is a curve of a focus signal obtained from the eight-segment photodetector shown in FIG. 6.

scription of the Inventio In the present invention, among the light beams travelling 7 the photodetector, the light beam around the central axis of the light travelling path, i.e., the light of the intermediate region between near- and f ar axis regions (having many components of spherical aberration) is blocked or shielded so that the light having fewer components of spherical aberration reaches the photodetector, thus stabilizing a focus signal. Thus, a disc drive which is compatible with discs having different thicknesses, e.g., 1.2mm compact discs and 0Amm digital video discs, is easily manufactured at low cost. The near-axis region represents the region around the central axis of the lens (defined as an optical axis in the optics) having a substantially negligible aberration. The far axis region represents the region which is f arther f rom the optical axis than the near-axis region, and the intermediate region is the region between the near-axis region and the f ar axis region.

FIG. 4 is a schematic diagram of an optical pickup device according to the present invention, in which light focusing on a thin disc and a thick disc are compared. Reference numerals 300a and 300b represent a thin disc (e.g., 0Amm digital video disc) and a thick disc (e.g., 1.2mm compact disc), respectively.

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

A focusing lens 800, a light controlling member 810 and a photodetector 820 are positioned along the light path of the light reflected from the beam splitter 700. The photodetector 820 is electrically connected to a magneto- 8 optical disc determiner 830. The photo-magnetic disc determiner 830 is connected to a differential amplifier 841 and an adder 842 for obtaining a photo-magnetic signal.

In the optical pickup -device according the present invention having the aforementioned configuration, the light controlling member 810 is made of a transparent material and has on its surface a light controlling f ilm 811 for absorbing, scattering or reflecting the light of the intermediate region between near- and far axis region having many components of spherical aberration.

In other words, as shown in FIG. 5, the light controlling film 811 interrupts (e.g., blocks, absorbs, scatters, diffracts, refracts or reflects) the light of the intermediate region between near- and far axis regions. Therefore, only the light beams of the near- and far axis regions reach the photodetector 820. The light controlling film 811 for blocking the light of the intermediate region may have various shapes such as an annular ring or perimetrical polygon (e.g., square or pentagon). Also, the light controlling film 811 may be provided as a coating film or a physical structure for blocking the light path.

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 positioned in the centre and a second detection region 822 divided into four parts is provided around the first detection region 821. The first detection region 821 includes four square light detecting elements Al, Bl, Cl and D1 and the second light detecting region 822 includes four L-shaped light detection elements A2, B2, C2 and D2.

The first detection region 821 is as large as a circumscribing square tangent to the light distributed region produced by the light passing through the inner side of the light controlling film 811 when the objective lens 9 is in focus with respect to the digital video disc 300a. At this time, the second detection region 822 is large enough to encompass all light beams incident beyond the light controlling film 811. This will be described with reference to FIGS. 7-12 for better understanding.

FIG. 7 shows the light distribution when the objective lens is in focus with respect to the digital video disc 300a. The light distributed region of the near axis light passing through the inner side of the light controlling film 811 is Internally tangent to the first detection region 821. The light passing through the outer side of the light controlling film 811 is distributed narrowly only in the second detection region 822.

FIG. 8 shows the light distribution when the objective lens is in a far-focus state with respect to the digital video disc 300a. At this time, as shown in FIG. 8, the light distribution lies horizontally, that Is, throughout the horizontal light receiving elements B2, Bl, D1 and D2.

FIG. 9 shows the light distribution when the objective lens is in a near-focus state with respect to the digital video disc 300a. At this time, as shown In FIG. 9, the light distributed region 820c is vertically elongated, that is, throughout the vertical light receiving elements A2, Al, Cl and C2.

FIG. 10 shows the light distribution when the objective lens 200 is in focus with respect to the compact disc 300b.

The light distributed region for the near axis light passing through the inner side of the light controlling 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 controlling film 811 is distributed widely in the second detection region 822 so that the second detection region 822 is internally tangent to the light distributed region for the light passing through the outer side of the light controlling film 811.

FIG. 11 shows the light distribution when the objective lens 200 is in a far-focus state with respect to the compact disc 300b. At this time, as shown in FIG. 11, the light distribution is horizontally elongated, that is, throughout the horizontal light receiving elements B2, Bl, D1 and D2. At this time, the light passing through the inner side of the light controlling film 811 Is also distributed in the first detection region 821 and the light passing through the outer side of the light controlling film 811 is distributed to be horizontally elongated in the second detection region 822.

FIG. 12 shows the light distribution when the objective lens 200 is in a near-focus state with respect to the compact disc 300b. In this case, unlike in FIG. 11, the light distributed region is vertically elongated, that is, throughout the vertical light receiving elements A2, Al, Cl and C2. However, at this time, the light passing through the Inner side of the light controlling film 811 is also distributed in the first detection region 821.

As described above, according to the present invention, 25 only the light passing through the inner side of the light controlling film 811, i.e., the near-axis light having small spherical aberration, reaches the first detection region 821.

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

FIG. 13 is a curve for comparing a focus signal S1 obtained 11 from the signal generated only f rom the f irst detection region 821 in a state where the light controlling film 811 as a feature of the present invention is used for the thick disc (compact disc) with a focus signal S2 obtained f rom all the signals generated from the first and second detection regions 821 and 822 where the light controlling film 811 is not used for the thick disc (compact disc). The width of the first detection region 821 for the photodetector used herein is set to 90pm and that of the second detection region 822 enclosing the first detection region 821 is set to 160pm.

Accordingly, as shown in FIG. 13, when using the compact disc 300b, if the light controlling film 811 is used and the first detection region 821 is used, a more stable focus signal S1 can be obtained, compared with the case when the first and second detection regions 821 and 822 are used. Also, when the compact disc is used, the far-axis light which exhibits a high degree of spherical aberration is widely distributed in the second detection region 822. Thus, the focus signal S2 is increased and the symmetry for the focusing direction can be maintained.

As described above, according to the optical pickup device 25 of the present invention, to read information from two discs having different thicknesses, i.e., a compact disc and a digital video disc, a light controlling film and an eight-segment photodetector are adopted so that only the near-axis light is received in the photodetector when the information is read from the compact disc and the near- and far-axis light is received in the photodetector when the information is read from the compact disc. Therefore, when the thick disc is used, a signal corresponding to the near axis region is obtained. When the thin disc is used, a relatively stable signal corresponding to both regions, i.e., the near and far axes, is obtained.

As described above, compared with the conventional optical pickup device, the optical pickup device according to the 12 present invention uses a light blocking or scattering means which is simple and easy to fabricate, e.g., a light controlling film formed on a transparent member or a light blocking or scattering groove formed on the objective lens; whereas the conventional optical pickup device adopts a complex and expensive hologram lens. Also, since the light is used without being separated by a hologram lens, the optical pickup device according to the present invention exhibits an improved light utilizing efficiency. Also, since a signal which can discriminate the disc type is obtained, a separate element is not required for discriminating the disc type.

13

Claims

CLAIMS:

1. An optical pickup device comprising:

a light source; an objective lens having a predetermined effective diameter and adapted to focus light from the light source towards the plane of a disc; a photodetector for detecting light reflected from the disc back through the objective lens; and light controlling means for interrupting the transmission of light of an intermediate region between near- and far axis regions.

2. A device according to claim 1 further including a light splitter for transmitting light from the light source to the objective lens and reflecting reflected light from the objective lens to the photodetector.

3. A device according to claim 1 or claim 2 in which the light controlling means comprises a separate transparent member.

4. A device according to claim 1 or claim 2 in which the light controlling means comprises a coating film for controlling the light emitted from the light source.

5. A device according to any preceding claim in which the photodetector includes a first detection region and a second detection region.

6. A device according to claim 5 in which the first and second detection regions are divided into multiple parts.

7. A device according to claim 5 in which the first and second detection regions of the photodetector are symmetrically divided into four parts and form a square.

8. A device according to any one of claims 5-7 in which the f irst detection region is an inner region and the 14 second detection region is a peripheral region.

9. A device according to any one of claims 5-8 in which the f irst detection region is of a size corresponding to near-axis light reflected from a thick disc and corresponding to near- and far-axis light reflected from a thin disc.

10. An optical pickup device substantially as described 10 herein with reference to FIGs. 4 et seq. of the accompanying drawings.