Method and apparatus for overwriting data on a magneto-optic disk

Abstract: Direct overwriting of encoded data along a track in a magneto-optic recording medium (10) is described. A laser (24) beam, not modulated by data, is directed at the active side of the medium to locally heat the width of one track for conditioning it for direct overwriting. A magnetic head (30) comprising an inductor and a capacitance in parallel with the inductor is movable in an appropriate manner to a selectable track. While the head is in alignment with the beam, current of a preselected polarity is supplied from a current source to the inductor. The current source is disconnected to reverse the field of the magnetic head by reciprocation of energy between the inductor and capacitance. Once the magnetic field is reversed, current is supplied to the inductor for maintaining the field until the next field reversal, the reversal times being determined by the data to be written. For pulse width modulation recording, the current normally supplied to the inductor is of one polarity and that supplied following the reversal is of opposite polarity; whereas for pulse position modulation recording, the current supplied to the inductor is of the same polarity before and after reversal. The inductor is preferably mounted for movement on a voice coil actuator and thereby maintained at a predetermined fixed distance from the medium.

Description: Technical Description This invention relates to a method and apparatus for direct overwriting of data on a magneto-optic disk without requiring modulation of a laser beam by data, and relates more particularly to such a method and apparatus that employs a unique circuit configuration for controlling field reversal of a magnetic head. Background of the Invention Direct overwriting is desirable to provide improved performance by not requiring a revolution of the disk for erasing data. It also provides better control of mark lengths relative to space lengths and permits higher linear densities through partial response signalling. To achieve direct overwriting in magneto-optic recording, the magnetic field of a magnetic head must be switched faster than the encoded bit rate. In systems where the magnetic head must be about a millimeter away from the active layer, the large volume of magnetic field needed for writing under such conditions requires high power to switch the field rapidly. The US patent US-A-4 872 078, assigned to the assignee of the present invention, describes a method whereby direct overwrite is achieved by a resonant circuit approach. Data may be directly overwritten by locally heating the active layer of the magneto-optic 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, marks of varying length can be written. This technique desirably permits multiple beam recording, in which each beam is properly synchronized with a global resonating field. However, this resonant coil technique requires synchronizing the laser pulses with the coil current; and mark edges cannot be placed arbitrarily, which leads to special code constraints that reduce the code rate and linear density. The Japanese Journal of Applied Physics, Vol. 26 (1987), supplement 26-4 at pp. 149-154 describes a technique whereby direct overwriting is achieved by use of a magnetic head which floats on an air-bearing slider, as in magnetic disk recording. The magnetic head produces a magnetic field that can be switched rapidly due to the low field volume. However, the circuitry for controlling the switching of the magnetic field is not disclosed. Moreover, a number of practical problems arise using this approach; e.g., aligning the pole piece with the focused beam, restriction of the technique to use with single-sided media, and the need for a clean environment to avoid head crashes. This reference cites, as prior art, a direct overwriting approach employing a fixed low-frequency magnetic head with wide spacing between the head and medium; and, because the head has to produce a large magnetic field to provide the field over a wide area, a large magnetic coil with a slow switching time must be used to apply an effective magnetic field to the medium. Thus, prior approaches to direct overwriting have involved either resonating the magnetic field with the electric field of a capacitor or reducing the field volume considerably by bringing the magnet close to the active layer. EP-A-312 143 discloses direct overwriting on a magneto-optical record carrier wherein the magnetic field is modulated according to the information to be recorded. The magnetic field modulator described comprises a field coil and a capacitance in parallel with the field coil. switching means allows for rapid reversal of the current through the field coil. There is a need for an improved method and apparatus for direct overwriting on magneto-optic media and which, in combination, provides the following advantages: (1) a circuit arrangement that permits high speed switching of the magnetic field with less energy than required by previously proposed techniques; (2) permits patterns to be written without code constraints; (3) does not require operation of the coil of a magnetic head in a resonant mode; and (4) does not require a magnetic head that flies above the media. Summary of the Invention The present invention is defined in the attached claims. Toward this end, and according to the invention, direct overwriting of encoded data along a track in a magneto-optic recording medium is achieved by directing a laser beam, not modulated by data, at a selectable track on the active side of the medium to locally heat the medium for conditioning it for direct overwriting. A magnetic head comprising an inductor and a capacitance in parallel with the inductor is movable in an appropriate manner to a selectable track. While the head is in alignment with the beam, current of one polarity is supplied from a current source to the inductor. The current source is disconnected to reverse the field of the magnetic head twice by reciprocation of energy between the inductor and capacitance. Current of the same polarity is supplied to the inductor for maintaining the field until the next data pulse. The inductor is preferably mounted for movement on a voice coil actuator and thereby maintained at a predetermined fixed distance from the medium. Brief Description of the Drawings Fig. 1 is a schematic diagram of an apparatus according to the invention which permits direct overwriting of data on a magneto-optic medium; Fig. 2 is a schematic diagram of a switching circuit illustrating the invention for direct overwriting on the medium using pulse position modulation (PPM) recording; Figs. 3A and 3B are voltage and current wave forms, respectively, for the circuit of Fig. 2; Fig. 4 is the preferred embodiment of the apparatus for direct overwriting using PPM recording. Description of Preferred Embodiment As illustrated in Fig. 1, a magneto-optic disk 10 is mounted on a spindle 12 that is rotatable by suitable means (not shown) and appropriately supported on a suitable stationary base 14. As illustrated, disk 10 comprises an active layer 16 interposed between a clear substrate 18 and a reflective layer 20 that is overcoated with a protective layer 22 -- a configuration that is conventional. A diode laser 24 directs a beam of collimated light via a mirror 26 and a focusing lens 28 onto the active layer 16. Mirror 26 and lens 28 are aligned with a magnetic head 30 that is adjacent the opposite (i.e., protective layer) side of disk 10. Mirror 26, lens 28 and head 30 are preferably mounted on a member 32, such as a voice coil actuator, that moves them in unison radially to a selected track on disk 10. Light projected by the laser beam onto the disk locally heats a region on the selected track to condition it for direct overwriting. Pulse Position Modulation Recording As illustrated in Fig. 2, a switching circuit for writing data using pulse position modulation (PPM) comprises a magnetic head 80. Head 80 comprises an inductor 82 and a capacitor 84 connected in parallel between a voltage source +V and a current source 86 by way of a normally closed switch 88. Assume that switch 88 has been closed long enough for the circuit to reach a steady state wherein all current is flowing through inductor 82. When a pulse is to be recorded, switch 88 is opened for 2 π LC seconds -- the time required for the energy to flow from inductor 82 to capacitor 84 and then back to inductor 82. The voltage and current wave forms are shown in Figs. 3A and 3B, respectively. Since switch 88 is closed when the voltage across inductor 82 is 0, no transient will occur. The inductor voltage and current stabilize at constant levels after each pulse in the data stream, and the process repeats for succeeding pulses. Typically, for each 1 bit in the data stream, a mark is written. Fig. 4 shows a preferred configuration of a circuit for writing data using PPM. This circuit comprises a high frequency bipolar transistor 100 consisting of an NPN transistor and a PNP transistor. When PPM data is supplied via a line 102 to the bases of the NPN and PNP transistors, an output signal is supplied via their commoned emitters to the gate of a FET 104. Magnetic head 106 comprises an inductor 108 and a capacitor 110 connected in parallel and interposed between a voltage source +V and a diode 112. Diode 112 is connected to the source of FET 104, the drain of which is connected to ground via a current source 114. FET 104 switches the current source 114. In operation, the voltage on line 102 is normally high, which energizes transistor 100. This, in turn, energizes gate G of n-channel FET 104. With gate G turned on, current flows via head 106, diode 112, FET 104, and resistor 114 to ground. When a mark is to be written on the disk, the voltage on line 102 goes low momentarily. This turns off bipolar transistor 100 and this, in turn, turns off FET 104. Diode 112 prevents the negative induced voltage on head 106 from reverse biasing FET 104. Head 106 is effectively disconnected from the rest of the circuit while the applied voltage to line 102 is low. While head 106 is disconnected, the energy reversal takes place between inductor 108 and capacitor 110. Theory The theory of the invention can most easily be demonstrated by reference to the LC circuit of Fig. 2, wherein the differential equation for the current flow is as follows: L d 2 i dt 2 + 1 C i = 0 The following initial conditions must be satisfied by the solution i (0 + ) = i (0 - ) i'(0 + ) = i'(0 - ) Condition (2) must be satisfied because the current flow in an inductor, such as inductor 82, must be continuous. Condition (3) must be satisfied because the voltage across the capacitor 84 (which is equal to the voltage across the inductor) must also be continuous. Suppose now that an initial current is flowing in inductor 82, but there is zero voltage across it. Then i(0)=I and i'(0)=0. The solution to the equation (i.e., the current behavior) is i t = I 0 cos 1 L C t where I is the initial current flow. This function i(t) describes the shapes of the wave forms of Figs. 3A and 3B during the time that the switches are off. It should be noted that laser 24 may be a continuous wave (CW) laser. However, the beam of laser 24 preferably is turned off momentarily during each field reversal to reduce noise during magnetic transitions and turned on only when the magnetic field is substantially maximum. Or, if preferred, the laser beam may be modulated at a constant high frequency in phase synchronization with field reversals to reduce noise during magnetic transitions and turned on only when the magnetic field is substantially maximum.

Claims: A method for direct overwriting of pulse position modulation encoded data on a magneto-optical recording medium of a magneto-optical storage apparatus which includes a laser, a magnetic head comprising an inductor and a capacitance arranged in parallel with the inductor for generating a magnetic field in the medium, and current source means for supplying current to said inductor; the method comprising the steps of: directing a laser beam, not modulated by data to be recorded, at a selectable track on the active side of the medium for locally heating the medium to condition it for direct overwriting; supplying current of a first polarity from the current source to the inductor; in response to a pulse in the data stream to be recorded, stopping the flow of the first polarity current to the inductor for a period of time during which the magnetic field of the magnetic head reverses twice by a reciprocation of energy between the inductor and the capacitance; and after the time period has elapsed, supplying current of the first polarity from the current source means to the inductor to maintain the magnetic field. Apparatus for direct overwriting of pulse position modulation encoded data on a magneto-optical data storage medium comprising: means (24) for directing a laser beam, non-modulated by data, at a selectable track on the medium to locally heat the medium to a condition for direct overwriting: a magnetic head (80) for generating a magnetic field to the medium comprising an inductor (8) and a capacitor (82) arranged in parallel with the inductor; current source means (86) for supplying the current of a first polarity; switch means (88) connected to the inductor and the current source means for supplying the first polarity current from the current source means to the inductor, the switch means being responsive to a data pulse to stop the flow of the first polarity current to the inductor for a period of time during which the magnetic field of the magnetic field of the magnetic field generator reverses twice by reciprocation of energy between the inductor and the capacitor, and to then supply the first polarity current from the current source means to the inductor. Apparatus as claimed in claim 2, further comprising a voice coil actuator on which the inductor is mounted for movement between tracks at a predetermined fixed distance from the medium.

Inventors: MAKANSI TAREK, MAKANSI, TAREK
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B11
IPC codes: G11B11