SYSTEM, METHOD AND INFORMATION CARRIER FOR LASER-STIMULATED MAGNETIC RECORDING

Abstract: A magnetic disk has a layered configuration comprising a recordable layer (110) and a reflective layer (112). Laser-stimulated recording takes place by a magnetic head (102) that obstructs the laser beam for a minor portion. The converging laser beam traverses the recording layer and gets reflected at the reflective layer, whereupon it reaches its focus in the recordable layer. The head gets only a fraction of the radiation. This enables to keep the head's temperature well within control.

Description: System, method and information carrier for laser-stimulated magnetic recording. TITLE OF THE INVENTION The invention relates to laser-stimulated magnetic recording of data on an information carrier. BACKGROUND ART Laser-stimulated magnetic recording allows for high-density magnetic recording. A small spot in the magnetic recording layer is heated by a laser beam in such a way that the amount of energy provided by the magnetic head and required for recording information in that spot, is substantially reduced with respect to a recording process at room temperature. Several configurations for magnetic head and optical head are known for implementing this laser-stimulated recording and are briefly discussed below. U.S. patent 5,565,385, incorporated herein by reference, discloses a configuration with a magnetic head located on one side of the recording medium and an optical head for the local heating located on the other side of the medium. This configuration requires two highly accurate, cooperating servo-systems for positioning the magnetic head and the optical head. U.S. patent 5,307,328, incorporated herein by reference, discloses another configuration wherein the laser head and magnetic head are positioned at the same side of the recording medium. This reference discloses a device for writing, reading or erasing a record carrier. The device has a coil with a winding arranged between an objective lens and the record carrier. Optical radiation is focused on a spot, the converging beam passing through the coil opening. A core of a transparent material is placed in the winding opening, the material having an index of refraction that significantly decreases the vergence of the focused beam. As a result the diameter of the coil can be reduced, and permits increase of the generated magnetic field. Although this known configuration works well within a certain parameter range, it has some drawbacks when the devices for writing and for reading are made smaller in order to attain higher data-densities on the record carrier. For example, the laser power per unit area squeezed through the transparent core increases with smaller device dimensions, thus potentially resulting in heating up the core, and eventually the magnetic head, so that the magnetic head's performance deteriorates as its temperature increases. U.S. 5,193,082, incorporated herein by reference, relates to yet another configuration. This reference discloses a system wherein a magnetic head and an optical head are integrated into a single unit. The unit comprises a single ferrite block. An optical system is integrated in a surface of the block that at one end carries the actual magnetic head. The optical system has a laser diode, a waveguide and a diffractor. The optical system and the actual magnetic head are located next to each other. Both the diffractor and the magnetic head face the recording medium. The diffractor focuses the laser beam in the waveguide at a point on the recording medium at which the magnetic head impresses a magnetic field. This configuration has some drawbacks as well. The unit in this configuration needs a relatively large ferrite block as a substrate. As a result, the maximum speed at which the unit can move is unduly determined by the block's inertia. Further, the laser beam is incident on the recording medium under a substantial angle. Any cross-section of the beam perpendicular to its axis is much smaller than the cross section of the illuminated spot on the medium, so that a more powerful diode is needed to attain the same energy density as with a perpendicular illumination. Also, the skewness of the laser beam incident on the medium's surface causes undesirable interference that may lead to canceling the light in one spot and amplifying it in another spot. This provides a non-uniform heating that is not well controllable and may even have negative effects on the recording medium's material. OBJECT OF THE INVENTION It is therefore an object of the invention to provide an alternative to the known recording approach. It is another object to provide a configuration so as to mitigate drawbacks inherent in the prior art. SUMMARY OF THE INVENTION To this end, the invention provides an information processing system for subjecting a recordable medium to a magnetic field and electromagnetic radiation for reading and/or modifying an information content of the recordable medium. The system comprises a magnetic head for generating the magnetic field; and a radiation source for generating the radiation. During operational use of the system the magnetic head is positioned substantially within the beam so as to cause the recordable medium to be subjected to the electromagnetic radiation that passes outside the magnetic head. For example, the uninterrupted beam has a disk-shaped cross-section. When the magnetic head is positioned within the beam, the cross- section of the beam is partly blocked by the head and has effectively a, e.g., substantially annular or substantially u-shaped cross-section in the plane of the head. Preferably, the optical path of the beam is such that the magnetic head does not absorb part of the electromagnetic radiation, but that the radiation is guided along the magnetic head. Since the magnetic head is small in comparison with the size of the beam cross-section, a focal point formed by the beam will have sufficient quality for the purpose of scanning a recording medium. Preferably, a reflective medium is provided in operational use of the system. The recordable medium is arranged between, on one side, the magnetic head and the radiation source and, on another side, the reflective medium. The reflective medium reflects the radiation to the side of the magnetic head so as to concentrate the radiation near the magnetic head upon being reflected and coming from the other side. The recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation, reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium, is much larger than the wavelength of the radiation. Preferably, recordable medium and the reflective medium are both integrated in a physical device, e.g., a magnetic disk, for cooperation with the system in operational use. The system may comprise a lens for controlling a main direction of the radiation, the magnetic head being mounted within the lens. The invention also relates to a physical device, e.g., a magnetic disk. The device has a recordable medium for being subjected to a magnetic field generated at a first side of the recordable medium and to electromagnetic radiation generated at the first side of the recordable medium for thereby modifying an information content of the recordable medium. The device has a reflective medium at a second side of the recordable medium, other than the first side, for reflecting the radiation towards the recordable medium. The recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium is much larger than the wavelength of the radiation. The recordable medium may comprise pre-recorded information that can be, e.g., supplemented or overwritten. The invention also relates to a method of processing information. The method comprises generating a magnetic field through a magnetic head, and generating electromagnetic radiation through a radiation source. A recordable medium is subjected to a combination of the magnetic field and the radiation for modifying an information content of the recordable medium. The subjecting comprises operating the magnetic head between the radiation source and the recordable medium so as to partly obstruct the cross-section through which the radiation passes. A reflective medium is used for reflecting the radiation, the recordable medium being positioned between the reflective medium and the magnetic head. The recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium is much larger than the wavelength of the radiation. The invention proposes to use a configuration with the laser and magnetic head at the same side of the recordable medium. A mirror is used at the opposite side of the recording medium such that the mirror receives the laser beam through the recordable medium and reflects it, thereby causing the reflected beam to be focused on or in the recording medium near the magnetic head. The mirror and the magnetic recording medium are preferably at a functionally constant distance from each other so that the length of the optical path is substantially constant. The path length is substantially larger than the wavelength of the radiation being used. The mirror is preferably physically integrated with a substrate, comprised in the mechanical carrier. The recording layer is put on, e.g., a thin sheet (thickness e.g., 0.1 mm- 0.2mm) of tightly controlled thickness. This sheet is attached to the substrate. The disk now has a mirror on one side of a thin substrate and a magnetic recording layer on the other side of the substrate. Such a substrate is then attached to a thicker and stronger carrier. As a result of above configuration, it is possible to focus the laser beam, after traversing the recordable layer and reflection at the mirror, on a spot of the recording layer that is opposite the magnetic head. The magnetic head now obstructs much less of the laser beam cross-section and, consequently, is not subjected to a fatally large amount of radiation as in some prior art configurations and, as a result, does not attain a temperature high enough to render the system inoperative. BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained further by way of example and with reference to the accompanying drawing wherein: Figs. 1, 2 and 3 are diagrams illustrating configurations of a system and recordable device in the invention. Throughout the figures, same reference numerals indicate corresponding or similar features. PREFERRED EMBODIMENTS Fig. 1 is a diagram of components in a magnetic recording system 100 of the invention. System 100 comprises a magnetic head 102, an optical sub-system 104 with a radiation source 106 for generating a beam of radiation 108, a recordable medium 110 suitable for recording or erasing information therein or thereon under combined control of magnetic head 102 and radiation source 106. Radiation source 106 comprises, e.g. a laser for using the principle of laser-induced magnetic recording as discussed above. Head 102 and source 106 are positioned on the same side of recordable medium 110, here the lower side. System 100 further comprises a reflective medium 112 to reflect the radiation incident on it from source 106 onto recordable medium 110. Magnetic head 102 is located in the optical path between radiation source 106 and recordable medium 110 so as to obstruct beam 108 only partly. Recordable medium 110 itself is positioned between magnetic head 102 and reflective medium 112. Reflective medium 112 serves to create a focal point 114 of the optical system at a location in recordable medium 112 just above (in the drawing) head 102. The optical path between source 106 and focal point 114 includes the portion from source 106 towards reflective medium 112, and the portion from reflective medium 112 to focal point 114. This optical path is longer than the length of the path from source 106 to the (in the drawing) lower side of recordable medium 110. Accordingly, the width of beam 108 in the plane of head 102 is much larger than a characteristic size of head 102, and the latter receives only a minor part of the radiation. Reflective medium 112 can be a stationary component relative to head 102 so that recordable medium 110 moves with respect to both head 102 and reflective medium 112. Alternatively, reflective medium 112 and recordable medium 110 are both parts integrated in a layered configuration such as a magnetic disk or tape. This is further explained with reference to Fig. 2. Fig. 2 is a diagram of a system 200 in the invention. System 200 comprises a magnetic disk 202 accommodating recordable medium 110 and reflective medium 112. Disk 202 comprises a layered configuration wherein recordable medium 110 is arranged between a protective layer 204 and a transparent layer 206. Reflective medium 112 is located between transparent layer 206 and a supporting substrate 208. The optical path length of the radiation between recordable medium 110 and reflective medium 112 is substantially longer than a typical wavelength of the radiation so that geometrical optics gives an adequate description of the paths. Preferably, the length of the optical path between recordable medium 110 and reflective medium 112 is substantially constant. This can be achieved by, e.g., arranging media 110 and 112 at substantially the same effective distance, and by having transparent layer 206 of a functionally uniform refractive index and of a functionally uniform thickness sandwiched in between. Reflective medium 112 can be deposited on substrate 208 using, e.g., chemical vapor deposition, or any other technology suitable for this purpose as known in the art. As a result of above configuration, it is possible to focus laser beam 108, reflected by mirror 112, on recording layer 110 above (in the drawing) magnetic head 102. Magnetic head 102 now obstructs much less of laser beam than in the known configuration mentioned above. Consequently, it is not subjected to fatally large amounts of radiation and, as a result, does not attain a temperature high enough to render system 100 or 200 inoperative. In a preferred implementation, radiation source 106 comprises a blue laser, having a wavelength in a range from e.g. 400 to 500 nm. Reflective medium 112 can be much thinner than, e.g., layer 206 that has a thickness of, for example, 0.1-0.2 mm. Optical subsystem 104 uses a high-NA lens, say an NA of 0.8 in order to read or write data in a functionally small area. For tracking purposes, an NA of 0.4 is already feasible. Layer 206 could be a thin sheet of well-controlled thickness such that the optical path through layer 206 (twice its thickness times its refractive index) is substantially constant. Layer 206 can be provided with pre-grooves at the magnetic recording layer for servo purposes (e.g., circular or spiral such as present in a CD or such as land groove structures as in DVD-R W/RAM. Systems 100 and 200 could be e.g., a consumer apparatus, a vending machine that enables a customer to create his/her own compilation of content (e.g., audio or video) by recording information selected by the customer in recordable medium 110. Fig. 3 is a diagram of an implementation 300 of systems 100 and 200. In implementation 300, magnetic head 102 is physically combined, e.g., merged within or mounted on, a lens 304 for focusing radiation beam 108. The positions of lens 304 and of magnetic head 102 are now controllable through a single actuator (not shown). The radiation beam 108 incident on lens 304 is an annular beam, i.e. the radiation is concentrated in an annular region, whereas substantially no radiation passes through the region 305 inside the annular region.

Claims: CLAIMS: 1. An information processing system (100) for subjecting a recordable medium (110) to a magnetic field and a beam of electromagnetic radiation (108) for modifying an information content of the recordable medium, wherein: - the system comprises: - a magnetic head (102) for generating the magnetic field; and - a radiation source (106) for generating the radiation; - during operational use of the system the magnetic head is positioned within the beam so as to subject the recordable medium to the electromagnetic radiation that passes outside the magnetic head. 2. The system of claim 1, wherein: - in operational use of the system a reflective medium (112) is provided; - the recordable medium is arranged between, on one side, the magnetic head and the radiation source and, on another side, the reflective medium; - the reflective medium is operative to reflect the radiation to the side of the magnetic head so as to concentrate (114) the radiation near the magnetic head upon being reflected and coming from the other side; and - the recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium is much larger than the wavelength of the radiation. 3. The system of claim 2, wherein the recordable medium and the reflective medium are both integrated in a physical device (202) for cooperation with the system in operational use. 4. The system of claim 3, wherein the physical device comprises a magnetic disk. 5. The system of claim 1, wherein: - the system comprises a lens (304) for controlling a main direction of the radiation; and o the magnetic head and the lens form one structural element. 6. A physical device (202) having: - a recordable medium (110) for being subjected to a magnetic field generated at a first side of the recordable medium and to radiation (108) generated at the first side of the recordable medium for thereby modifying an information content of the recordable medium; and - a reflective medium (112) at another side of the recordable medium other than the first side for reflecting the radiation towards the recordable medium; wherein: - the recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium is much larger than the wavelength of the radiation. 7. The device of claim 6, comprising a magnetic disk. 8. The device of claim 6, wherein the recordable medium comprises pre-recorded information. 9. A method of processing information comprising: - generating a magnetic field through a magnetic head (102) ; - generating radiation through a radiation source (106); - subjecting a recordable medium (110) to a combination of the magnetic field and the radiation for modifying an information content of the recordable medium, wherein the subjecting comprises: - operating the magnetic head between the radiation source and the recordable medium so as to partly obstruct the radiation; - using a reflective medium (112) for reflecting the radiation, the recordable medium being positioned between the reflective medium and the magnetic head; wherein: - the recordable medium and the reflective medium are arranged so that a length of an optical path of the radiation reflected at the reflecting medium to a region of concentration of the radiation in the recordable medium is much larger than the wavelength of the radiation.

Inventors: FAESSEN LOUIS M H, VISSER DERK, FAESSEN, LOUIS, M., H., VISSER, DERK
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: G11B11
IPC codes: G11B5, G11B11, G11B13