Magneto-Optical Recording and Reproducing System

The present invention relates to a recording and reproducing device for use with a magneto-optical recording medium, and more particularly to a magneto-optical recording and reproducing device which is arranged so that it is able to simultaneously perform a plurality of operations such as erasing, recording and reproducing and to make up the shortage of power for erasing and recording.

Conventional magnetic recording and reproducing devices employing a magneto-optical recording medium are roughly divided into the one-beam type and the three-beam type. The former performs recording and reproducing with one beam and also uses the same beam for tracking. In contrast thereto, the latter employs two beams for tracking use separately of a beam for recording and reproducing and performs tracking servo based on a difference between the two beams as illustrated in U.S. Pat. No. 3,905,040 to Otala. Either system uses the same beam for recording and reproducing. Recording takes place by irradiating the magneto-optical recording medium by a laser beam to raise the temperature of the irradiated portion, reducing its coercive force to cause a reversal of magnetization of that portion to the direction of the magnetic field. On the other hand, reproducing is performed by irradiating the recording medium by the same beam weakened to such an extent as not to destroy the record and detecting the rotation of the plane of polarization of reflected or transmitted light from the medium in accordance with the direction of its magnetization. The magneto-optical recording medium makes a great feature of arbitrary erasing and restoring of records. According to the prior art system, in the case of storing new information in a recording medium having already recorded thereon information, a portion of a recording track where the new information is to be recorded is entirely scanned by a beam in the presence of a homogeneous magnetic field perpendicular to the surface of the medium to lower its coercive force, directing the magnetization of the medium to the same direction as the ambient magnetic field. This corresponds to erasing. Next, a magnetic field opposite in direction to that employed for erasing is applied to the recording medium to store thereon the new information on the aforementioned principle. For checking whether or not the information has been correctly recorded, it is necessary to reproduce the entire recorded information. The conventional system thus employs the same beam for all of the erasing, recording and reproducing operations, and consequently it has the defect that only one operation can be performed at one time.

An object of the present invention is to provide a magneto-optical recording and reproducing system which is designed so that a plurality of beams are used in an optical system and one servo system to permit two or three of the erasing, recording and reproducing operations to be performed concurrently and that the shortage of power can be filled up by a plurality of beams in the case of using a recording medium requiring a high laser power for recording or erasing.

In accordance with the present invention, there is provided a magneto-optical recording and reproducing system comprising: a magneto-optical recording medium having a magnetization vector perpendicular to its film surface; optical system means for generating a plurality of beams through the use of light from a plurality of light sources; mode magnetic field generation means for generating a mode magnetic field taking either a forward or backward direction along the beams at a plurality of irradiating positions where the beams are applied to the magneto-optical recording medium; and a drive mechanism for driving the optical system and the mode magnetic field generating means or the magneto-optical recording medium so that the plurality of irradiating positions may shift on the magneto-optical recording medium.

The present invention will hereinafter be described in detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram explanatory of a conventional magneto-optical recording and reproducing system;

FIGS. 2A and 2B are system diagrams illustrating embodiments of an optical arrangement of the present invention;

FIG. 3 is a graph showing the characteristic of a mode magnetic field for use in the present invention;

FIG. 4 is a schematic side view illustrating a specific example of mode magnetic field generating means for use in the present invention;

FIG. 5 is a schematic diagram explanatory of an example of the distribution of magnetic fluxes of mode magnetic fields set up by the mode magnetic field generating means in the present invention;

FIG. 6 is a characteristic diagram explanatory of an example of a magnetic field gradient of the mode magnetic field generated by the mode magnetic field generating means for the use in the present invention;

FIG. 7 is a schematic diagram showing stored bits explanatory of the operation of this invention system;

FIG. 8 is a block diagram illustrating an embodiment of the present invention; and

FIGS. 9 and 10 are characteristic diagrams explanatory of the operation of this invention system.

With reference to FIG. 1, a conventional three beam system employs two beams 2 for tracking use separately of a beam 1 for recording and reproducing. This conventional system has the above-mentioned defects.

While in the following the present invention will be described mainly in connection with the case of two beams for convenience' sake, the invention is also applicable to the case of three or more beams.

The erasing, recording and reproducing operations using a laser beam call for a complex optical system and servo system for an automatic focusing control mechanism and an automatic tracking mechanism and, accordingly, also in case of performing such operations by a plurality of beams, it is inefficient if the optical and servo systems are not used in common to them. Various forms can be considered for utilizing an optical system with a plurality of beams; two examples are shown in FIGS. 2A and 2B. These examples are arranged so that beams 3 and 4 are brought by a half mirror 5, a total reflection mirror 8 and an objective lens 7 to focus to adjacent positions on the same circumference on a disc surface 11, and that their reflected lights are taken out in a desired direction through the half mirrors 6 and 10 and a total reflection mirror 9.

Combinations of those two of the three operations of erasing, recording and reproducing which are likely to be concurrently performed are as follows:

(i) Recording-Reproducing

(ii) Reproducing-Erasing

(iii) Erasing-Recording

The combination (i) can be achieved, for example, by recording with a first beam in a state in which a mode magnetic field for determining the mode of operation is applied to the recording medium in an upward direction and then reproducing the record by irradiating the recording medium by a second beam of a fixed intensity which is so low as not to destroy the record. (This irradiation will hereinafter be referred to as the DC irradiation.) In the case of the combination (ii), reproducing is effected by the DC irradiation with a first beam of low intensity in the presence of a downward mode magnetic field and then erasing is carried out also by the DC irradiation with a second beam. In this way, the combinations (i) and (ii) can be achieved with ease.

In contrast thereto, the combination (iii) presents a problem. This is because the mode magnetic fields necessary for erasing and recording are opposite in direction to each other. Accordingly, the opposite mode magnetic fields must be applied to two spots on the disc which are sometimes spaced only several micrometers apart. To perform this, it is necessary to impart a spatially steep field gradient to the mode field as shown in FIG. 3 so that opposite mode magnetic fields may be applied to the two spots. This can easily be achieved by disposing two electromagnets 12 and 13 in the front of and at the back of the two spots as shown in FIG. 4 and by flowing a current to them so that the opposite mode magnetic fields may be applied to the spots, respectively. FIG. 5 schematically shown magnetic fluxes emanating from the two electromagnets.

Further, since the field intensity in the vicinity of each electromagnet can be considered to be inversely proportional to the square of the distance from the electromagnet, the magnetic field distribution between these electromagnets becomes such as shown in FIG. 6; this indicates that such a steep magnetic field gradient as depicted in FIG. 3 can be obtained. It is a matter of course that an actual arrangement is equipped with the function of adjusting the positions of the electromagnets so that the boundary between the upward and downward magnetic fields may lie intermediate between the two spots.

It is apparent that the magnetic field generating mechanism, though described above to employ the electromagnets, is not limited specifically thereto but may also use, for example, a permanent magnet capable of changing the direction of a magnetic field.

Since erasing is carried out by the DC irradiation, there is no possibility of records being left unerased in the track direction but the erasing beam may deflect slightly in the radial direction. If the erasing beam has exactly the same diameter and power as those of the recording beam, there is the likelihood that if the beam deflects when erasing bit trains 14 stored as shown in FIG. 7, only recording areas 15 are erased but leaving the others 16 unerased. In view of this, it is necessary to somewhat strengthen the power of the erasing beam so that the stored areas can be erased with a margin.

The following Table 1 is a summary of the operations (i) to (iii) described above. For convenience' sake, the direction of the mode magnetic field is upward in the case of recording, and the beam which is applied first to the recording medium is referred to as the beam 1. It is needless to say that in an actual device, such changeover is automatically set by turning a switch to positions (i) to (iii), respectively.

The operation (i) on the Table 1 is carried out as follows:

(1) The switching circuit 19 selects the following operation modes:

(a) The beam 1 is reset to "recording" while the beam 2 "reproducing".

(b) The powers of the beam 1 and the beam 2 are established to be "medium" and "weak", respectively, as indicated on the Table 1.

(c) In connection with the irradiation system, the beam 1 is modulated by the modulator 17 while the beam 2 is held constant for DC radiation regardless of the modulator 17.

(d) The upward mode magnetic field or the downward mode magnetic field generated by the electromagnets 12 and 13 in the magnetic field generator 24 is reset for "recording". The direction of the mode magnetic field has no direct relation to the reproduction operation since it uses the reflected light beams from the recording medium 27 in response to the beam 2.

(2) The electric signal input of the modulator 17 is modulated by the modulator 17 so that the beam 1 for the recording operation is ON-OFF controlled. The beam 1 ON-OFF controlled is applied to the recording medium 27 through the optical system 23. Information of the electric signal input is stored on the recording medium 27 by applying the upward or downward magnetic field in accordance with the ON-OFF states of the beam 1.

(3) In case of the reproducing, mode of operation, the beam 2 of DC weak power is applied on the recording medium 27 through the optical system 23, so that the reflected light beam is applied, through the optical system 23, to the photo detecting system 22 (including two detectors). In this photo detecting system 22, a rotation of the plane of polarization of the reflected light beam is detected to read out the directions of the mode magnetic field stored on the recording medium 27. The readout signal is transferred through the switching circuit 19 to the demodulator 18 to demodulate into an output information signal 0.

The details of the operation (ii) are omitted since they can be understood from the former description for the combination (ii) Reproducing-Erasing in view of the above operation (i).

In the operation (iii), there is the possibility that before the temperature of the medium 27 raised by the first erasing beam has not sufficiently lowered, a mode magnetic field opposite to that during erasing is applied to the medium 27 to cause a reversal of the magnetization of the medium 27 regardless of whether the beam for recording is ON or OFF. Assuming that the temperature of the recording medium 27 is raised by the first erasing beam up to T.sub.2 in FIG. 9 and that the temperature lowers to T.sub.1 when the spot comes under the recording beam, and letting the coercive forces of the recording medium 27 at the temperatures T.sub.1 and T.sub.2 be represented by H.sub.C1 and H.sub.C2, respectively, the abovesaid phenomenon can be prevented by controlling a bias magnetic field so that H.sub.C1 <H<H.sub.C2.

In this multi-beam system, it is also possible to make positive use of heat left by the first beam irradiation when recording or erasing with a recording medium of high coercive force. In FIG. 10, in a case where due to a power shortage of the first beam, the temperature of the recording medium rises only to T.sub.2 below a temperature T.sub.W at which recording is possible and the temperature drops to T.sub.1 when the irradiated spot comes under the second beam, its power can be used to fill up the shortage of power for raising the temperature by .DELTA.T=T.sub.W -T.sub.1, thereby permitting recording or erasing.

In the foregoing, known driving means may be employed for moving the optical system and the mode magnetic field generating means or the recording medium relative to each other so that the beam may shift on the recording medium.

The foregoing description has been given of the case where the beam spacing is considered to be fixed but, if the beam spacing is made variable, then a higher power can be obtained by applying a plurality of beams to exactly the same spot when filling up the aforesaid shortage of power.

As has been described in the foregoing, the magneto-optical recording and reproducing device of the present invention is capable of simultaneously performing two or three of the erasing, recording and reproducing operations by the use of a plurality of beams which is impossible with the prior art system. Further, the device of the present invention also permits the use of a recording medium of high coercive force by filling up a power shortage with a plurality of beams when recording or erasing.