Optical pickup device

Abstract: An optical pickup device which can obtain a stable reproduction signal by an increased numeric aperture of an objective lens (210). The optical pickup device includes an objective lens unit (200) having an objective lens (210) facing a disc (300) and having a predetermined effective diameter and at least two auxiliary lenses (230) provided in exterior sides of the objective lens (210) for directing +1st-order diffracted light to a light path different from that of the light passing through the objective lens (210), a light source (800) for irradiating light onto the disc (300) through the objective lens (210), a light splitter (400) interposed between the light source (800) and the objective lens (210) for directing the light reflected from the disc (300) to a light path different from that from the light source (800), a photodetector for detecting the light reflected from the disc and travelled via the light splitter (400), and at least two diffracted light detectors (610A, 610B) for detecting diffracted light incident from the auxiliary lenses (230). Therefore, a high reproduction signal and a stable track signal can be obtained.

Description: The present invention relates to an optical pickup device, and more particularly, to an optical pickup device which can obtain a stable reproduction signal by an increased numeric aperture of an objective lens. An optical pickup records and reproduces information such as video or audio data onto/from optical recording media, e.g., discs (or disks). A disc has a structure that a 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 (NA) 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 , the magnitude of a coma aberration coefficient W 31 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. Therefore, considering that the numerical aperture 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, 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 therefore controlled to be 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. From a users' standpoint, it is very important to equip a high-density disc with compatibility with a conventional disc. However, the change in the disc thickness due to the high density results in a corresponding spherical aberration. 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. Finally, a compatible pickup which can read discs having different thicknesses, such as a CD or a DVD, becomes necessary. The conventional compatible pickup utilizes a hologram lens (as in Japanese Patent Laid-open Publication No. hei 7-98431), adopts two objective lenses, or adopts a physical aperture diaphragm (as in U.S. Patent No. 5,281,797). However, since such conventional compatible pickups require additional components, the manufacturing cost is increased accordingly. EP-A-0190698 describes the reproduction of an optically recorded signal using interference between reflected diffracted light of +1 st order or -1st order and reflected diffracted light of 0 th order coercively interfere to enable a high frequency signal to be reproduced. EP-A-0351953 describes an optical head with tilt correction servo-mechanism in which optical heads of an optical recording and reproducing device are provided with a sero-mechanism for correcting the tilting of the optical axis of a light beam with respect to the recording surface of an optical disc. The arrangement of EP-A-0351953 has an objective lens and a photodetector positioned in a linear light path and a light source positioned in a light path diffracted by a light splitter. US 5446565 describes a compound objective lens having two focal points composed of a hologram lens for transmitting part of incident light without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of first-order diffracted light, and an objective lens for converging the transmitted light to form a first converging spot on a front surface of a thin type of information medium and converging of the diffracted light to form a second converging spot on a front surface of a thick type of information medium. The Japanese patent application JP-A-04228124 which forms the basis of the preamble of claim 1 describes an optical pickup in which light from a semiconductor laser is transmitted through the inside of an optical wave guide and converged on an optical disc. The 0 th order reflected light at the disc becomes a leading optical wave and is converged on a photodiode to provide an output signal. The +/- 1 st -order diffracted light beams which are generated by the diffraction of the pit on the optical disc are converged onto further photo diodes to become output signals. The Research Disclosure Number 231 published on 1 January 1991 describes at page 49 a light corrector for correcting light scattered at an angle larger than the maximum incident light angle and focussing it onto a read detector. The arrangement offers a high signal to noise ratio. Also, the high-density disc such as the DVD has a track pitch and pit much smaller than those of the conventional CD. Further, since the degree of modulation of the DVD is reduced by more than 35% compared to that of the CD, it is difficult to obtain a stable track signal. Particularly, in the case of the DVD, if an incident light beam deviates from the centre with respect to the objective lens during reproduction of information, i.e., if a beam shift occurs, an optical modulation function cannot exceed a diffraction limit curve. It is an aim of embodiments of the present invention to provide an improved optical pickup which can obtain a stable reproduction signal and servo signal by detecting ±1st-order diffracted light which is reflected from a disc as a track compensation signal, which can adopt both an objective lens having a light blocking means and a conventional objective lens. According to a first aspect of the invention, there is provided an optical pickup device comprising: an objective lens unit having an objective lens facing a disc and having a predetermined effective diameter and at least two auxiliary lenses provided in exterior sides of said objective lens the auxiliary lenses arranged in use to direct ±1st-order diffracted light to a light path different from that of the light passing through said objective lens; a light source for radiating light onto a disc through said objective lens; a light splitter interposed between said light source and said objective lens for directing the light reflected from said disc to a light path different from that from said light source; a photodetector for detecting the light reflected from the disc and travelling via said light splitter; at least two diffracted light detectors to detect diffracted light incident from said auxiliary lenses; and characterised by: first light controlling means provided between said objective said auxiliary lenses to control the light of the intermediate region between near- and far-axis regions of incident light and provided around a central axis of said objective lens, the near-axis region comprising the region around the central axis of said objective lens having substantially negligible aberration, the far-axis region being the region farther from the central axis and the intermediate region being between the near- and far-axis regions; and, second light controlling means provided in the light passing region of the objective lens to control the light of the intermediate region so that information can be written to or read from two disc types having different thicknesses. Said light source objective lens may be positioned in a linear light travelling path and said photodetector positioned in the path diffracted by said light splitter. Alternatively, said objective lens and photodetector may be positioned in a linear light path and said light source positioned in the path diffracted by said light splitter. Preferably, said auxiliary lenses are hologram lenses based on a diffraction theory. Said objective lens and auxiliary lenses may be integrally formed. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic diagram of an optical pickup device; Figure 2 is a schematic diagram of another optical pickup device; Figure 3 is a schematic front view of an objective lens; Figure 4 is a schematic side view of an objective lens adopted for an optical pickup device according to an embodiment of the present invention; Figure 5 is a plan view of a four-segment photodetector adopted for optical pickups according to embodiments of the present invention; Figures 6 and 7 show spot shapes formed on the four-segment photodetector shown in Figure 5, depending on disc type; Figure 8 is a plan view of an eight-segment photodetector adopted for optical pickups according to embodiments of the present invention; Figures. 9 to 11 show spot shapes formed on the eight-segment photodetector shown in Figure 8, depending on the distance between an objective lens and a thin disc; Figures. 12 to 14 show spot shapes formed on the eight-segment photodetector shown in Figure 8, depending on the distance between an objective lens and a thick disc; and Figure 15 is a graph illustrating the light modulation function for optical pickup devices according to embodiments of the present invention. Referring to Figure 1, an objective lens unit 200 facing a disc 300 has an objective lens 210 with a light controlling means 220 in its periphery, and an auxiliary lens 230 positioned in the outer area of the light controlling means 220. A photodetector 600 for detecting the light passing through the objective lens 210 is positioned in the light path of the objective lens 210. A beam splitter 400 and a detecting lens 500 are in turn positioned between the objective lens unit 200 and the photodetector 600. A light source 800 such as a laser diode is positioned in the path diffracted by the beam splitter 400. Auxiliary photodetectors 610A and 610B for detecting the light from the auxiliary lens 230 are provided in exterior sides of the photodetector 600. In such a configuration, as the auxiliary lens, a hologram lens based on a diffraction theory as well as a general refractive lens may be adopted. Figure 2 is a schematic diagram of an optical pickup device. In this device, in contrast with the first device, a light source 800 and an objective lens unit 200 are positioned in a linear light travelling path. As shown in Figure 3, an objective lens unit 200 has an objective lens 210, a light controlling means 220 surrounding the objective lens 210, and an auxiliary lens unit comprising two auxiliary lenses 230 disposed around the light controlling means 220. Auxiliary photodetectors 610A and 610B for detecting the light emitted from the auxiliary lenses 230 are positioned in exterior sides of the light source 800. A wavelength plate 900 and a polarizing splitter 400a are positioned between the light source 800 and the objective lens unit 200. The photodetector 600 is positioned in the path diffracted by the polarizing splitter 400a. A detecting lens 500 is interposed between the polarizing splitter 400a and the photodetector 600. A collimating lens 801 for focusing the light emitted from the light source 800 is positioned between the polarizing splitter 400a and the light source 800. An auxiliary detecting means is provided in exterior sides of the light source 800. In each of the above two devices, the light emitted from the light source 800 and the zero-order diffracted light reflected from the disc 300 pass through the objective lens 210, and the ±1st-order diffracted light reflected from the disc 300 pass through the auxiliary lens 230. At this time, the ±1st-order diffracted light partially passes through the edge portion of the objective lens 210. Therefore, the zero-order diffracted light reflected from the disc 300 and having passed through the objective lens 210 reaches the photodetector 600 via the beam splitter 400a. The ±1st-order diffracted light having passed through the auxiliary lens 230 reaches the auxiliary photodetectors 610A and 610B. The zero-order diffracted light detected from the photodetector 600 and the ±1st-order diffracted light detected from the auxiliary photodetectors 610A and 610B are summed in an adder (not shown) to then be used as a reproduction signal. A difference signal of the -1st-order diffracted light detected from one auxiliary photodetector 610A with respect to the +1st-order diffracted light detected from another auxiliary photodetector 610B is used as a track signal. As described above, an optical pickup device adopting the principles described above, allows a more stable control signal and reproduction signal to be obtained by the use of the fist-order diffracted light reflected from the disc 300, compared to the conventional optical pickup device utilizing only the light passing through one objective lens as a signal source. In the aforementioned optical pickup device, the auxiliary lens and auxiliary photodetector can be installed anywhere, without being limited by the above-described examples. Also, the objective lens, auxiliary lens and light controlling means may be manufactured of any suitable material. The objective lens unit 200 includes a second light controlling means for controlling, i.e., blocking, scattering or absorbing, the light of the intermediate region between the near axis and far axis, among the light beams passing through the objective lens 210, so that information can be written or read from two discs having different thicknesses. The optical pickup device according to the present invention, which can be commonly used for two discs, is equipped with an objective lens unit (described later) and its overall structure is such that the objective lens unit and photodetector in the first and second devices are replaced with an objective lens unit and a photodetector to be described later, respectively. Figure 4 schematically shows an objective lens unit adopted to an optical pickup device according to an embodiment of the present invention. In the objective lens unit, there is provided a first light controlling means 220 surrounding an objective lens 210 having a predetermined numerical aperture, e.g., 0.6, and an auxiliary lens unit 230 disposed around the light controlling means 220, which is the same as the structure of the objective lens unit 200 of the first embodiment. In addition, in this embodiment, there is provided a second light controlling means 240 of an annular shape having a predetermined diameter in the light passing plane of the objective lens 210. The second light controlling means 240 controls, i.e., blocks, scatters or absorbs, the light of the intermediate region between the near- and far-axis regions among the light beams passing through the objective lens 210. In this embodiment, the light around the central axis of the light path, i.e., in an intermediate region between near-axis and far-axis regions, is controlled, i.e., blocked or shielded to form a small light spot from which interference of the light in the intermediate region is suppressed. To this end, in the intermediate region between the near axis and the far axis along the incident light path, there is provided light controlling means of an annular or perimetrical polygon (e.g., square) shape for controlling, i.e., blocking or scattering, light. This utilizes the fact that the light of the far-axis region does not affect the central light but the light of the intermediate region does. Here, the near-axis region is the region around the central axis (optical axis) of the lens having a substantially negligible aberration, the far-axis region is the region farther from the optical axis, and the intermediate region is between the near- and far-axis regions. In the optical pickup device according to this embodiment of the present invention, adopting the objective lens unit 200, there is provided a photodetector for selectively detecting the light passing through the second light controlling means 240 when information is read from a thick disc, e.g., a 1.2mm CD, and for detecting the light passing through the objective lens 210 when information is read from a thin disc, e.g., a 0.6mm DVD. As such a photodetector, the conventional four-segment photodetector may be adopted, as shown in Figure 5. A spot formed in the center of a photodetector 600 having four detecting elements 601, 602, 603 and 604 is shown in Figures 6 and 7, depending on the disc thickness. Figure 6 shows the shape of a spot formed on the photodetector 600 when adopting a relatively thick disc, e.g., a 1.2mm CD. Figure 7 shows the shape of a spot formed on the photodetector 600 when adopting a relatively thin disc, e.g., a 0.6mm DVD. The central portion 651 of the spot produced by the near-axis region light undergoes a small change in diameter, irrespective of the disc thickness. However, the diameters of the intermediate portion 652 and peripheral portion 653 are changed greatly. In other words, as shown in Figure 6, the central portion 651 of the spot is positioned in the center of the photodetector 600 and the peripheral portion 653 thereof surrounds the photodetector 600. The intermediate portion 652 between the central portion 651 and peripheral portion 653 is a light-eliminated portion by the second light controlling means. In other words, when information is reproduced from a thick disc, a 1.2mm disc, the central portion 652 and peripheral portion 653 are greatly enlarged by spherical aberration, so that only the light of the near axis region, i.e., the central portion 651 of the spot, is used. Also, as shown in Figure 7, the central portion 651 and peripheral portion 653 are both formed on the detection plane of the photodetector 600. In other words, when information is reproduced from a thin (0.6mm) disc, all of the light of both the near- and far-axis regions, excluding the light of the intermediate region which is removed by the light controlling member, are used. Here, the diameter of the central portion 651 is maintained to a similar level, irrespective of a disc type. As described above, in order to read information from two discs having different thicknesses, the optical pickup device according to embodiments of the present invention adopts the 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-axis 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 high signal corresponding to the light of the near- and far-axis regions is obtained. The photodetector 600 may be of another type, as shown in Figure 8. The photodetector 600 shown in Figure 8 has a structure such that second detection region is added around the photodetector of Figure 5. In other words, the photodetector 600 in this embodiment includes the first detection region 611 which is located centrally and the second detection region 612 located around the first detection region 611. The first detection region 611 consists of four square first-light-receiving elements A1, B1, C1 and D1. The second detection region 612 consists of four L-shaped second-light-receiving elements A2, B2, C2 and D2 disposed so as to surround the first-light-receiving elements A1, B1, C1 and D1 of the first detection region 611. Figures. 9-11 show the light-receiving states of the photodetector when a thin disc (DVD) is used. Figures 12-14 show the light-receiving states of the photodetector when a thick disc (CD) is used. In the photodetector having the aforementioned structure, the entire signal, i.e., that from both the first and second detection regions, is used in reading information from a thin disc, and only the signal from the first detection region is used in reading information from a thick disc. As described above, according to the optical pickup device of the present invention, a zero-order diffracted light signal detected from the photodetector 600 and a +1st-order diffracted light signal detected from the auxiliary photodetector 610A or 610B are summed in an adder (not shown) to be used as a reproduction signal. Also, a difference signal of a -1st-order diffracted light signal detected from another auxiliary photodetector 610B with respect to a +1st-order diffracted light signal detected from one auxiliary photodetector 610A is used as a track signal. Therefore, a high reproduction signal and a stable track signal can be obtained. As the result, as shown in Figure 15, a light modulating function value (B) of the optical pickup device indicative of the reproduction signal is considerably increased, compared to that (A) of the conventional optical pickup device. Also, since there is provided in an objective lens a second light controlling means for controlling the light between the near and far axes regions, a 1.2mm CD and a 0.6mm DVD can be commonly used without any configuration change.

Claims: An optical pickup device comprising: an objective lens unit (200) having an objective lens (210) facing a disc (300) and having a predetermined effective diameter and at least two auxiliary lenses (230) provided in exterior sides of said objective lens (210), the auxiliary lenses (230) arranged in use to direct ±1st-order diffracted light to a light path different from that of the light passing through said objective lens (210); a light source (800) for irradiating light onto said disc (300) through said objective lens (210); a light splitter (400) interposed between said light source (800) and said objective lens (210) for directing the light reflected from said disc (300) to a light path different from that from said light source (800) ; a photodetector (600) for detecting the light reflected from said disc and travelling via said light splitter; at least two diffracted light detectors (610A, 610B) to detect diffracted light incident from said auxiliary lenses (230) ; characterised by : a first light controlling means (220) provided between said objective lens (210) and said auxiliary lenses (230) to control the light of the intermediate region between near- and far-axis regions of incident light and provided around a central axis of said objective lens (210), the near-axis region comprising the region around the central axis of said objective lens having substantially negligible aberration, the far-axis region being the region farther from the central axis and the intermediate region being between the near- and far-axis regions; and a second light controlling means (240) provided in the light passing region of the objective lens (210) to control the light of the intermediate region so that information can be written to or read from two disc types having different thicknesses. An optical pickup device as claimed in claim 1, wherein said light source (800)and objective lens (210) are positioned in a linear light travelling path and said photodetector (600) is positioned in the path diffracted by said light splitter (400). An optical pickup device as claimed in claim 1, wherein said objective lens (210) and photodetector (600) are positioned in a linear light path and said light source (800) is positioned in the path diffracted by said light splitter (400). An optical pickup device as claimed in claim 1, 2 or 3, wherein said auxiliary lenses (230) are hologram lenses based on a diffraction theory. An optical pickup device as claimed in claim 1, 2 or 3, wherein said objective lens (210) and auxiliary lenses (230) are integrally formed.

Inventors: LEE CHUL-WOO, YOO JANG-HOON, LEE, CHUL-WOO, YOO, JANG-HOON
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1996
Main classification: G11B7
IPC codes: G02B13, G11B7