Hologram optical pick-up using two laser sources

Abstract: Disclosed is a hologram optical pick-up using two laser sources (11, 11'). In a hologram optical pick-up according to the present invention, laser beams having different wavelength, which are emitted from first and second laser sources (11, 11') and focused by an objectiveive lens (16) on a disc (17, 18), are reflected at a surface of the disc (17, 18). The reflected laser beams are diffracted by a first hologram (13) or a second hologram (14), and then received by a first photo detector (19) or a second photo detector (20). Accordingly, laser beams having different wavelengths according to a thickness and a recording surface of the disc are emitted by the first laser source (11) or the second laser source (11') to compensate for aberration of laser beam due to the thickness of the disc (17, 18). Thereby, the hologram optical pick-up can reproduce data from discs (17, 18) having different recording surface without loss of data. Also, the hologram optical pick-up can record and reproduce data on/from any disc under best environment and can increase efficiency of the laser beam.

Description: The present invention relates to an optical pick-up capable of recording and reproducing data on and from discs having different thickness, and more particularly to a hologram optical pick-up capable of recording and reproducing data on and from any kind of disc by using two laser sources emitting laser beams having different wavelength, thereby improving efficiency of laser beams. A compact disc is well known as a recordable and reproducible optical medium. The compact disc generally is a substrate having a thickness of 1.2 mm and has a reflecting surface as a recording layer. Information such as a sound, a character, a graphic, and the like, are recorded in a form of pits on the reflecting surface. A digital video disc having a thickness of 0.6 mm of a substrate also has been developed as one of the recordable and reproducible optical medium. Amount of information including image data are stored with high density in the digital video disc. An optical pick-up for the digital video disc has many differences from an optical pick-up for the compact disc. The optical pick-up of the digital video disc includes a laser diode emitting laser beams having a short wavelength and a objective lens having a high numerical aperture. Thereby, the optical pick-up of the digital video disc collects the laser beams and focuses the laser beams in a form of fine spots on a disc to reproduce data recorded with high density from the disc. On the other hand, recently, a phase changeable disc has been developed on which data are recorded only once and reproduced therefrom. The phase changeable disc has the same thickness as that of the compact disc and has a different recording layer from the compact disc. That is, the compact disc has pits arranged as a data recording layer formed on a surface thereof, but the phase changeable disc has phase changeable materials, by which a reflecting ratio of the laser beams during recording data is different from a reflecting ratio of the laser beams during reproducing data in a range of certain wavelength regions. The optical pick-up can record and reproduce the data on such phase changeable discs and compact discs under same environment. As discs which have different thickness or recording material have been developed, an optical pick-up capable of using all of the different types of disc is required by users. According to the requirement, there has been provided an optical pick-up capable of recording and reproducing data on and from both of a digital video disc and a compact disc. The optical pick-up capable of recording and reproducing the data on and from both of the digital video disc and the compact disc generally uses a laser diode emitting laser beams having a short wavelength, for example a wavelength of 650 nm, and can adjust a numerical aperture of an objective lens according to the thickness of a disc to be reproduced. That is, during reproducing data from the digital video disc, the optical pick-up focuses laser beams in a form of fine spot on the digital video disc by using the objective lens with a numerical aperture of 0.6, and during reproducing data from the compact disc, the optical pick-up makes diameters of laser beams, which are incident from the laser diode to the objective lens, to be small or makes laser beams to pass through a portion, for example a short axis portion at which the numerical aperture is 0.45, at which the numerical aperture is lower in the objective lens. Thereby it is possible to compensate an aberration of the laser beams. In the optical pick-up applying to both of the digital video disc and the compact disc as described above, a surface emitting laser diode is used as a laser source, and a hologram is used as an element adjusting the numerical aperture of the objective lens. Referring to Figure 1, a hologram plate 2 having first and second holograms 3 and 4 at both sides thereof is positioned in front of a laser source 1. A collimating lens 5 and an objective lens 6 are disposed in order in front of the hologram plate 2. The laser source 1 emits laser beams. The laser beams pass through the hologram plate 2, which in turn are modulated into parallel beams by the collimating lens 5. Then, laser beams are incident into and focused by the objective lens 6 on a digital video disc 7 or a compact disc 8. The laser beams are reflected by the digital video disc 7 or the compact disc 8 and pass again through the objective lens 6 and the collimating lens 5. Then, the laser beams are diffracted by the holograms 3 and 4 and detected by first and second photo detectors 9 and 10. During reproducing data from the digital video disc 7, the laser beams passing through the first hologram 3 are focused in a form of fine spot on the digital video disc 7 by using the numerical aperture of the objective lens 6. The reflected laser beams are diffracted by the first hologram 3 and received by the first photo detector 9. The first photo detector 9 changes intensity of the laser beams into electric signals. On the other hand, when reproducing data from the compact disc 8, laser beams are made to pass through the second hologram 4 having a small area and the portion at which the numerical aperture of the objective lens 6 is lower. Thereby, it is possible to compensate for the aberration of the laser beams generated due to the thickness difference between the digital video disc and the compact disc. Then, the laser beams are focused in a form of spots on the compact disc 8. The laser beams are reflected by the compact disc and diffracted by the second hologram 4, which in turn are detected by the second detector 10. In the conventional optical pick-up as described above, however, since the optical pick-up uses the laser source emitting the laser beams which have the short wavelength so as to focus the laser beams in the form of fine spots on the digital video disc, there is a disadvantage in that when the optical pick-up reproduces data from a phase changeable disc, the data can be destroyed due to properties of a phase changeable material. Furthermore, in the conventional optical pick-up, the optical environment generally is adjusted for the digital video disc which, having a thinner thickness, presents the problem in that it is impossible to optimally reproduce data from the compact disc and that during the reproduction of data from the compact disc, a loss of the laser beams which arrives at the photo detector increases to result in deteriorating the reproducing signal. Therefore, the present invention has been made with a view to overcome or reduce the above described problems of the prior art, and accordingly it is an aim of preferred embodiments of the present invention to provide an optical pick-up which has an improved hologram, for recording and reproducing data on and from discs having different thickness and discs, for example, a recordable compact disc, an magneto optical disc, and the like, having different recording material. It is another aim of embodiments of the present invention to provide an optical pick-up using a hologram, capable of recording and reproducing data on and from any disc having different thickness under suitable environment. According to a first aspect, the present invention provides a hologram optical pick- up using two laser sources comprising: first and second laser sources for respectively emitting laser beams having different wavelengths; an objective lens for focusing the laser beams emitted from the first and second laser source on surfaces of discs having different thickness; a hologram plate having a first hologram on one side, EP747893 filed on 5-6-96 and published on 11-12-96 shows an optical pick-up of that type, however the present invention further provides a second hologram formed on the other side thereof, the first and second holograms respectively diffracting the laser beams which are emitted from the first and second laser sources and reflected by discs; and first and second photo detectors for receiving the laser beams which are reflected by the discs and diffracted by the first and second holograms so as to detect signals. The first laser source preferably emits a laser beam having a wavelength of 670 nm and the second laser source emits a laser beam having a wavelength of 780 nm. The discs having different thickness may include a digital video disc and a compact disc or a digital video disc and a recordable compact disc. Preferably, the first and second laser sources and the first and second photo detectors are disposed on the same surface. Preferably, the first hologram only diffracts the reflected laser beam of the first laser source corresponding thereto and the second hologram only diffracts the reflected laser beam of the second laser source corresponding thereto. 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 constructional view of a conventional optical pick-up using a hologram; and Figure 2 is a schematic constructional view of an optical pick-up according to a preferred embodiment of the present invention, in which holograms are used for diffracting laser beams. A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Figure 2 schematically illustrates an optical pick-up according to a preferred embodiment of the present invention. The hologram optical pick-up has two laser diodes for respectively emitting laser beams having different wavelengths as laser sources. When recording and reproducing the data on and from disc, the hologram optical pick-up selects a laser diode having a suitable wavelength for the disc. As shown in Figure 2, the hologram optical pick-up includes first and second laser diodes 11 and 11', a hologram plate 12 having first and second holograms 13 and 14 at both sides thereof, a collimating lens 15, an objective lens 16, and first and second photo detectors 19 and 20. Reference numerals 17 and 18 indicate discs having different thicknesses from each other. A thinner disc 17 is a digital compact disc, and a thicker disc 18 is a compact disc or a recordable compact disc. Of course, only one of discs 17 and 18 is loaded into the optical pick-up apparatus in order for the hologram optical pick-up to record and reproduce data on and from the selected disc. The first and second laser diodes 11 and 11' respectively emit laser beams having different wavelengths. That is, the first laser diode 11 emits a laser beam having a wavelength of 670 nm for the digital video disc 17 and the second laser diode 11' emits a laser beam having a wavelength of 780 nm for the compact disc or the recordable compact disc. The first and second laser diodes 11 and 11' are disposed on the same surface together with the first and second photo detectors 19 and 20. A transparent substrate such as glass, is used as the hologram plate 12 and has the first and second holograms 13 and 14 formed at both sides thereof. The first and second holograms 13 and 14 respectively diffract the laser beams when the laser beams respectively emitted from the first and second laser diode 11 and 11' are reflected by the surface of discs. That is, the first hologram 13 diffracts the laser beam which is emitted from the first laser diode 11 and reflected by a surface of the thinner disc 17 such as the digital video disc. At this time, the laser beam which is emitted from the second laser diode 11' and reflected by the surface of the thinner disc 17 passes through the first hologram 13 without diffraction. In the case of diffracting the laser beam by using the second hologram, the second hologram 14 diffracts the laser beam which is emitted from the second laser diode 11' and reflected by a surface of the thick disc 18 such as the compact disc. At this time, the laser beam which is emitted from the first laser diode 11 and reflected by the surface of the thick disc 18 passes through the second hologram 14 without diffraction. An objective lens 16 having a high numerical aperture is used in the hologram optical pick-up in order to form a spot on the surface of the thinner disc 17. When the objective lens 16 is used to form the spot on the surface of the thick disc 18, an aberration of the laser beams is increased due to the relative thickness difference of the thinner disc and the thick disc. The increasing of the aberration of the laser beams can be prevented by reducing an area of the second hologram 14 or by using a laser diode having a small emitting angle as the second laser diode 11'. In the hologram optical pick-up according to the embodiment of the present invention as described above, while recording and reproducing data on and from the thinner disc 17 such as the digital video disc, the first laser diode 11 having a short wavelength is used to record and reproduce the data on and from the thinner disc 17. While recording and reproducing data on and from the thick disc 18 such as the compact disc or the recordable compact disc, the second laser diode 11' having a long wavelength is used to record and reproduce the data on and from the thick disc 18. In the case of reproducing the data from the thinner disc 17, the laser beams emitted from the first laser diode 11 pass through the first and second hologram 13 and 14 of the hologram plate 12 and are modulated into parallel beams by the collimating lens 15. Then, the laser beams are focused by the objective lens 16 on the surface of the disc 17 with a spot and reflected by the surface of the disc 17. The reflected laser beams pass again through the objective lens 16 and the collimating lens 15 and diffracted by the first hologram 13. Then, the laser beams are detected by the first photo detector 19. In the case of reproducing the data from the thick disc 18, the laser beams emitted from the second laser diode 11' pass through the first and second hologram 13 and 14 of the hologram plate 12 and are modulated into parallel beams by the collimating lens 15. Then, the laser beams are focused by the objective lens 16 on the disc 18. At this time, the laser beams pass through a portion, for example a short axis portion, at which the numerical aperture of the objective lens is small. Thereby, the aberration due to the thickness difference of the disc 17 and disc 18 is compensated. The laser beams are reflected by the surface of the disc 18. The reflected laser beams pass again through the objective lens 16, the collimating lens 15, and the first hologram 13. Then, the laser beams are diffracted by the second hologram 14 and detected by the second photo detector 20. According to the embodiment as described above, a disadvantage in that the aberration is increased due to the thickness difference of the disc 17 and 18 can be overcome by using two laser diodes having difference wavelengths. Furthermore, the hologram optical pick-up can reproduce the data from the recordable disc having different recording substrate, such as a phase changeable disc, without loss of the data. Accordingly, the hologram optical pick-up described has an advantage in that kinds of discs having different thicknesses can be used in the hologram optical pick-up. The hologram optical pick-up according to the present invention has an increased performance by providing a suitable optical environment for the discs. The holograms respectively diffract the laser beams having different wavelength, resulting in minimizing the loss of the laser beams, preventing a deterioration of the data, and improving a diffraction efficiency of the laser beams. While the present invention has been particularly shown and described with reference to the particular embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the scope of the invention as defined by the appended claims.

Claims: A hologram optical pick-up using two laser sources comprising: first and second laser sources (11, 11') for respectively emitting laser beams having different wavelengths; an objective lens (16) for focusing said laser beams emitted from said first and second laser sources on surfaces of a disc (17, 18) having one of different thicknesses; a hologram plate (12) having first and second holograms (13, 14) formed on both sides thereof, said first and second holograms (13, 14) respectively diffracting said laser beams which are emitted from said first and second laser sources (11, 11') and reflected by said discs (17, 18); and first and second photo detectors (19, 20) for receiving said laser beams which are reflected by said discs (17, 18) and diffracted by said first and second holograms (13, 14) so as to detect signals. A hologram optical pick-up using two laser sources as claimed in claim 1, wherein said first laser source (11) emits a laser beam having a wavelength of 670 nm and said second laser source (11') emits a laser beam having a wavelength of 780 nm. A hologram optical pick-up using two laser sources as claimed in claim 2, wherein discs having different thickness include a digital video disc (17) and a compact disc (18). A hologram optical pick-up using two laser sources as claimed in claim 2, wherein discs having different thickness include a digital video disc (17) and a recordable compact disc (18). A hologram optical pick-up using two laser sources as claimed in any of the preceding claims, wherein said first and second laser sources (11, 11') and said first and second photo detectors (19, 20) are disposed on the same surface. A hologram optical pick-up using two laser sources as claimed in any of the preceding claims wherein said first hologram (13) only diffracts said reflected laser beam of said first laser source (11) corresponding thereto and said second hologram (14) only diffracts said reflected laser beam of said second laser source (11') corresponding thereto.

Inventors: KIM JIN-HWAN, SHIN HUN-KUK, KIM, JIN-HWAN, SHIN, HUN-KUK
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1997
Main classification: G11B7
IPC codes: G11B7