METHOD AND APPARATUS FOR ENCODING AND DIRECT OVERWRITING OF MAGNETO-OPTIC DATA

Abstract: Old data is erased and new data is written during a single rotation of the media (10) in a data storage system which includes a resonant coil (14) for producing a high frequency alternating magnetic field interacting with laser pulses (16) having a duration equal to a small fraction of the period of the magnetic field. Variable-length magnetic domains are formed by overlapping circular magnetic domains to provide pulse width modula­ tion (PWM) encoding of data.

Description: This invention relates to magneto-optic data storage devices, more particularly, to writing new encoded data over old data in devices of that kind. Presently, most magneto-optic data storage devices require one rotation of the disk to erase old data before new data can be written. This arrangement is used because of the difficulty to switch the polarity of the magnetic field rapidly enough to correspond to the new data stream to facilitate directly overwriting old data. USP 3,676,867 teaches an arrangement and interaction of a laser and magnetic-field-producing coil for magnetizing a thin film in an easy direction of magnetization normal to the substrate on which the thin film is deposited. The magnetic field, being weaker than the coercive force of the film, is used for erasing and creating reversed domains in the film at points which are simultaneously heated by a laser beam. USP 4,340,914, USP 4,495,530 and USP 3,778,791 disclose methods in which the magnetic field is controlled or modulated by a signal representing data. USP 4,495,530 actually teaches away from using a electromagnet for producing a bi-polar magnetic field because of perceived high frequency limitations of such an arrangement. Instead, two other systems configurations for processing information recorded on a magneto-optic medium are described. In one configuration, described for example in 'Erasable Magneto-Optical Recording' Philips Technical Review Vol. 42, No. 2, August 1985, pages 37-47, a pulsed laser is used with a steady-state DC magnetic field. In the other, the magnetic field is driven by the data signal which interacts with a continuous wave laser. Finally, in USP 4,472,748 describes a system in which a constant erasing light beam and a modulated recording light beam are focused on adjacent tracks and a reversible external magnetic field is applied to the recording medium. The polarity of the erased tracks alternates every other track. The magnetic field is reversed by switching the direct current supplied to the magnetic-field-generating coil at each revolution of the disk. USP 4,466,004 discloses a technique whereby a continuous laser heating source interacts with a data modulated magnetic field. Data is recorded in the magneto-optic medium when the polarity of the applied magnetic field corresponds to the polarity of the code for the desired data. The forming of circular domains with a laser beam in magnetically coercive materials is known from, for example USP 3,786,452, 3,836,895, 3,787,825 and 3,836,897; however, use of patterns of such domains, including overlapping of successive circular domains to form variable-length elongated domains, for recording encoded data is not shown. The present invention provides high linear bit density, substantial immunity to domain size variation and good erasability in a magneto-optic data storage system. In addition, since data may be directly overwritten, overall system speed is enhanced. According to the present invention, data may be directly overwritten in a magneto-optic medium by locally heating a domain in the active layer of the medium with a short pulse of laser light in the presence of a periodically alternating magnetic field produced by a high frequency resonant coil. By selecting the duration of the laser pulse to be a small fraction of the period of the alternating magnetic field and controlling the timing of the pulse with respect to the phase of the alternating magnetic field, encodable circular and variable-length elongated magnetic domains can be written in the medium. When the locally heated domain cools, that domain in the medium is magnetized in either up or down direction normal to the plane of the substrate in which the medium is deposited as determined by the phase of the alternating magnetic field. In the present invention, pulse-width modulation (PWM) is used to encode data. Such modulation is produced by controlling the distance between the walls of magneto-optically polarized domains along a track in the medium. Such domains, according to the present invention, may be controllably elongated by overlapping circular domains produced by firing the laser synchronously with the oscillating magnetic field. The overlapped circular domains form continuous domains of variable-length. Fig. 1 is a schematic cross-sectional view of a magneto-optic recording system according to the principles of the present invention. Fig. 2 is a timing diagram showing the timing of laser pulses with respect to the phase of applied magnetic field for the system of Fig. 1. Figs. 3A, 3B and 3C are schematic diagrams of resonant circuits for producing a magnetic field for the system of Fig. 1. Fig. 4 is a timing diagram showing the formation of encodable variable-length magnetic domains for the system of Fig. 1. Fig. 5 is a timing diagram showing the encoding of data for the system of Fig. 1. Fig. 6 is a track diagram showing the immunity to spot-size variation for the system of Fig. 1. Referring to Fig. 1, a magneto-optic recording system according to the principles of the present invention, comprises recording medium 10 having active layer 12, a source of laser light 13, under the control of laser controller 19, for producing pulsed laser light beam 16 focussed on active layer 12 by lens 18, and a source of alternating magnetic field 14 for producing an alternating magnetic field at megahertz frequencies in the vicinity of the area in active layer 12 at which laser beam 16 is focussed. Laser beam 16 is used to locally heat a domain in active layer 12 in the presence of alternating magnetic field 20 to form a magnetized domain of either up or down direction normal to the plane of medium 10. Detector 15, which includes well-known optics and electronic circuitry for detecting laser light reflected back from layer 12, produces a readback signal therefrom which represents recorded data. As shown in Fig. 2, the laser beam is applied to active layer 12 in short pulses the duration of which being a small fraction of the period of alternating magnetic field 20. When the medium cools after the laser pulse terminates, the medium is magnetized in the direction of applied magnetic field at the time the laser pulse was applied. For example, if laser pulse 16 were applied to active layer 12 during time Δt₁, the polarity of the magnetized domain in the active layer is in the down direction with respect to the plane of active layer 12. Similarly, if laser beam 16 were applied to active layer 12 during time Δt₂, the magnetization at the domain would be in the up direction with respect to the plane of active layer 12. Magnetic field 20 may be produced any number of ways. According to the present invention, however, to achieve megahertz frequencies, a resonant coil circuit is used. Three embodiments of resonant coil circuits are shown in Figs. 3A-3C. In Fig. 3A, a parallel resonant circuit provides a high input impedance which can be matched to a 50 ohm driver by a step-up transformer. In Fig. 3B, a series resonant circuit, having low input impedance can be matched to driver impedance using a step-down transformer. In Fig. 3C, a parallel resonant circuit with a tapped coil provides impedance matching and required resonant magnetic fields. Component values are determined by the desired operating frequency of magnetic source 14. Employing the circuit of Fig. 3C, an alternating magnetic field of 200 gauss peak-to-peak can be produced by a coil wound on a rod-like ferrite core. When driven at approximately 25 watts of input power, the circuit, which had a Q of approximately 30 and an input impedance of 50 ohms, resonated at a frequency of 2.3 megahertz. With such a circuit acting as magnetic source 14 disposed approximately one millimeter away from active layer 12, encodable magnetic domains are produced in active layer 12 with 40 nanosecond duration pulses from laser light source 13. Laser light source 13 may be any commonly available laser, such as a gallium arsenide diode laser. Fig. 4 illustrates a method of encoding data which is compatible with the high frequency magnetic field direct-overwrite system of the present invention. The distance between magnetic domain walls formed in active layer 12 is used to encode the data which is a form of PWM encoding. Continuous elongated domains are formed in active layer 12 by activating laser source 13 several times, once per period during corresponding phases of the alternating magnetic field. Each laser pulse forms a circular domain. For a high enough laser pulse rate and low enough disk velocity, successively formed circular magnetic domains of the same magnetization direction may be disposed in overlapping relationship to form continuous elongated domains of variable-length. Referring now to Fig. 4, if the frequency of magnetic field 20 is f B and the radius of a single domain formed in active layer 12 is r, successively formed circular domains will overlap if d is less than 2r, where d is the maximum distance between successive circular domains. For good erasability and low signal noise, a high percentage of overlap of one domain over another is desirable. Even so, the edges of the resulting elongated domain are not straight, tending to be "scalloped". The fraction of unwritten area, u, compared to an idealized domain with straight edges may be approximated by u = 1/2√ [1- 1 - d²/4r²] If d=r, then u is 6.0%, which is a measure of overlap imperfection. Imperfect overlap and resulting unwritten area may cause high frequency noise to be present in the detected signal. However, noise can be negligible if the value of 1/d is near the spatial frequency cutoff of the optical detection system or if the domain width is substantially larger than the optical beam size. In addition, the magnetic properties of the medium may act to smooth out the submicron domain width variations leading to further reduction in noise. The overlap imperfection can also be reduced by making d smaller. For example, if d=0.5r, the u is reduced to 1.6%. The requirement for good domain overlap sets a limitation on disk velocity. If d=r is the desired overlap parameter, then disk velocity is given by v=rf B . In converting user data to encoded data, pulse crowding and clock synchronization must be considered. In magnetic recording, these considerations are characterized by the concept of (d,k) parameters in run length limited (RLL) codes. The same parameters are applicable to the magneto-optic direct overwrite recording system of the present invention if further constrained by requiring that the number of consecutive zeroes in (d,k) codes be even. Referring to Fig. 5, laser source 13 under the control of laser controller 19 may be activated twice during the period of the magnetic field. Thus, the frequency of clock pulses is 2f B . Up-oriented domains may be formed by activating a laser on odd clock pulses, and down- oriented domains may be formed on even clock pulses. Therefore, down-to-up transitions may be formed only at odd clock pulses. Similarly, up-to-down transitions may be formed only at even odd clock pulses. Since a change in magnetic orientation at domain walls within a clock period represents a binary 1 while no such change during a clock period represents a zero, an already "up" oriented domain must be changed to a "down" oriented domain to produce a binary 1 (and conversely). Thus, as shown in Fig. 5., transitions are necessarily separated by an even number of clock pulses, and binary 1's in (d, k) constrained RLL codes applicable to the present invention are separated by an even number of zeroes. The size of a basic circular domain found in magneto-optic media can vary as a result of many factors including laser power, media sensitivity, ambient temperature, magnetic field strength, focussing error and contamination on the substrate and in the optical subsystem. Referring now to track (a) of Fig. 6, in traditional pulse width modulation encoding, the domains are formed using a single orientation of magnetic field. If the diameter of the basic circular domain increases for some reason, then the length of the encoded domain undesirably elongates along the recording track. If the length change is large enough in a high density storage device, detection errors will arise during data decoding. In the present invention, the distance between domain transitions along the recording track is substantially unaffected by a change in the diameter of the basic circular domains comprising the elongated domains as shown at track (b) of Fig. 6. If the diameter of the basic circular domains changes, the overlapping relationship of adjacent up-and down-oriented domains formed in active layer 12 according to this invention tends to cancel elongation of such domains along the recording track. See, for example, that l 1a does not equal l 2a in track (a) domains, but l 1b substantially equals l 2b in track (b). Thus, the distance between transitions will be insensitive to variations in the size of the basic circular domains.

Claims: Apparatus for direct overwriting of encoded data along a track in a magneto-optic recording medium, said apparatus comprising means (14) for producing a high frequency magnetic field having periodically alternating magnetic polarity near the track; and pulsed heating means (13,19) coupled to the magnetic field producing means, for selectively applying heat to said medium to form successively overlapping magnetically polarised domains representing encoded data along the track, heat being selectively applied at such times when the instantaneous polarity of the magnetic field at said domains corresponds to the polarity of the code for the data to be recorded thereat. Apparatus as in claim 1, wherein the magnetic polarity of the magnetic field alternates at frequency f B ; and the pulsed heating means may be activated at a frequency of approximately 2f B . Apparatus as in claim 2, wherein the pulsed heating means may be activated at least once during each period of the alternating magnetic field. Apparatus as in claim 2, wherein: each of said successively overlapping domains is substantially circular; and the relative velocity, v, of the track with respect to the magnetic field producing means and the pulsed heating means is given by the relation v ≦ rf B , where r is the radius of one of said overlapping domains. Apparatus as in claim 4, wherein the maximum distance between successive domains is less than 2r. Apparatus as in claim 5, wherein the distance between successive domains is approximately equal to r. Apparatus as in claim 6, wherein the distance between successive domains is approximately equal to 0.5r. Apparatus as in claim 1, wherein: the magnetic field producing means includes a resonant coil; and the pulsed heating means includes a selectively controllable source of laser light. Apparatus as in claim 1, wherein said successively overlapping domains form a series of selectively variable-length, substantially continuous, elongated magnetically polarized domains for encoding data in pulse width modulation codes. Apparatus as in claim 9, wherein the encoded data represented by said elongated domains is substantially unaffected by variation in the size of said successively overlapping domains. Apparatus as in claim 10, wherein each of said successively overlapping domains is substantially circular; and the length of said elongated magnetically polarized domains along the track is substantially unaffected by variation in the diameter of each of said successively overlapping substantially circular domains. Apparatus as in claim 9, wherein the pulse width modulation code is run length limited; and the number of consecutive zeroes in said code is even. A method for direct overwriting of encoded data along a track in a magneto-optic recording medium, said method comprising the steps of providing a high frequency magnetic field having periodically alternating magnetic polarity near the track; and selectively applying heat to said medium to form successively overlapping magnetically polarized domains representing encoded data along the track, heat being selectively applied at such times when the instantaneous polarity of the magnetic field at said domains corresponds to the polarity of the code for the data to be recorded thereat. A method as in claim 13, wherein the step of providing a magnetic field includes the step of alternating the polarity of the magnetic field at frequency f B ; and the step of selectively heating includes the step of selectively heating domains at a frequency of approximately 2f B . A method as in claim 14, wherein the step of selectively heating includes the step of selectively heating domains at least once during each period of the alternating magnetic field. A method as in claim 14 further including the steps of: forming successively overlapping, substantially circular domains; and moving the track with respect to the first and second means at a relative velocity, v, given approximately by the relation v ≦ rf B , where r is the radius of one of said domains. A method as in claim 16, wherein the step of forming successive domains includes the step of forming successive domains having a distance apart of less than 2r. A method as in claim 17, wherein the step of forming successive domains includes the step of forming successive domains having a distance apart of approximately r. A method as in claim 18, wherein the step of forming successive domains includes the step of forming successive domains having a distance apart of approximately 0.5r. A method as in claim 13 further including the step of forming a series of selectably variable-length substantially continuous, elongated, magnetically polarized domains for encoding data in pulse width modulation codes. A method as in claim 20 further including the step of encoding data represented by said elongated domains which is substantially unaffected by variation in the size of said successively overlapping domains. a method as in claim 21 further including the steps of: forming successively overlapping, substantially circular domains; forming elongated magnetically polarized domains, the length of which along the track being substantially unaffected by variation in the diameter of each of said successively overlapping, substantially circular domains. A method as in claim 20 further including the steps of: encoding data in pulse width modulation code which is run length limited; and further constraining said code so that the number of consecutive zeroes in said code is even.

Inventors: GERBER, ULRICH GERHARD, RUGER, DANIEL
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: G11B13
IPC codes: G11B11