Apparatus for generating magnetic field in a magneto-optic recording/reproducing system

In an apparatus for generating a magnetic field, first and second electromagnetic coils are linearly arranged on a yoke. The coils face one surface of an optical disk. An optical head is positioned above the other surface of the optical disk. A track address for denoting a track on the optical disk is picked up from the optical disk by the optical head and an information signal processor. A first and/or second energizing signal is generated from a switching circuit in accordance with the track address. Thus, one or both of coil drive circuits are driven in response to the energizing signals, energizing one or both of the coils, so that a predetermined magnetic field is applied to the optical disk from the selected coil or coils.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for generating a magnetic 
field in a magnetooptic recording/reproducing system utilizing a 
magnetooptic effect and, more particularly, to an apparatus for generating 
a magnetic field for applying a magnetic field to an information recording 
medium to record or erase the information in a magnetooptic 
recording/reproducing system for recording or erasing the information on 
the medium by utilizing a magnetooptic effect. 
In a magnetooptic recording/reproducing system utilizing a magnetooptic 
effect, a laser beam is converged onto an information recording medium 
(hereinafter referred to as "an optical disk") and a magnetic field is 
applied to a region on the optical disk irradiated with the laser beam in 
a recording or erasing mode. Therefore, the magnetooptic 
recording/reproducing system has an apparatus for generating a magnetic 
field for applying a magnetic field to the optical disk. A conventinal 
apparatus for generating a magnetic field is disclosed in Japanese Patent 
Laid-open Publication No. 119507/1984. The apparatus disclosed in this 
Publication has a magnetic yoke comprising a pair of bar sections and a 
base section on which the bar sections are formed, and the magnetic yoke 
has a length equal to the width of a region scanned with a laser beam in a 
radial direction of the optical disk. As shown in FIG. 1, in a 
magnetooptic recording/reproducing system, optical head 11 is disposed 
above one side surface of optical disk 10, and magnetic unit 13 is 
disposed under the surface of disk 10. In case of accessing a 
predetermined track on disk 10, head 11 is moved in the radial direction R 
of disk 10, as shown in FIG. 1 to record information on a desired track, 
to reproduce information from a desired track or to erase information 
recorded on a desired track. In a recording and reproducing mode, a 
vertical magnetic field is applied from unit 13 to disk 10. Unit 13 has 
bar sections 14B and 14C formed on base section 14A and an exciting coil 
15 wound around one section 14B of magnetic yoke 14 which has a length 
equal to a width of a region to be scanned with a laser beam in a radial 
direction of disk 10. 
Since a vertical magnetic field is applied to the entire region to be 
scanned with the laser beam in the radial direction of the optical disk by 
such magnetic unit 13, coil 15 increases in size and consumes a large 
amount of power and exciting coil 15 and its driving circuit generates a 
relatively large heat emission, thereby resulting in a high cost system. 
Since the vertical magnetic field is applied to not only the area 
irradiated with the laser beam but the other areas irradiated with no 
laser beam, another drawback arises that the efficiency of the magnetic 
field to be applied to the beam irradiated area per exciting power is 
wrong. 
A magnetic unit for solving the above-mentioned drawbacks is disclosed in 
Japanese Patent Laid-open Publication No. 40761/1982. This magnetic unit 
13 has, as shown in FIG. 3, one base section 14A having a length equal to 
a width to be scanned with a laser beam in a radial direction of an 
optical disk and a bar section 14B formed on the base section 14A, two 
segments 17A, 17B separated from each other and arranged along bar section 
on base section 14A, and exciting coils 18A, 18B respectively wound around 
divided segments 17A, 17B. A vertical magnetic field Hy having a level Hc 
or higher as shown in FIG. 4 is applied onto optical disk 10 by unit 13 in 
recording or erasing mode. In FIG. 4, when applying a vertical magnetic 
field H.sub.1 to first region Z.sub.1 of disk 10, first coil 18A is 
selected and merely excited. When applying a vertical magnetic field to 
second region Z.sub.2 of disk 10, second coil 18B is selected and merely 
excited. When applying a vertical magnetic field H.sub.3 to third region 
Z.sub.3 between regions Z.sub.1 and Z.sub. 2, one of coils 18A and 18B is 
selected and energized. 
In unit 13 shown in FIG. 3, there arise drawbacks that a leakage magnetic 
flux occurs between segments 17A and 17B of yoke 14 as shown in FIG. 5 and 
intensity Hy of the vertical magnetic field decreases to level Hc or lower 
at the ends of regions Z.sub.1 and Z.sub.2 as shown in FIG. 4. As a 
result, intensity Hc of a magnetic field sufficient to record or erase 
information is not applied to a boundary region on disk 10 facing a space 
region between segments 17A and 17B, and information may not be stably 
recorded nor reliably erased. In order to solve this problem, it is 
considered that a relatively large current should always be supplied to 
the selected coil to generate a vertical magnetic field having sufficient 
intensity Hc. Since a large current must be supplied to the coil, the 
result is a large amount of heat generated in the coil and large power 
consumption. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an apparatus for 
generating a magnetic field which can generate a magnetic field having 
sufficient intensity to all regions to be scanned with a light beam on an 
information recording medium with low power consumption and low heat in 
high utility efficiency of the magnetic field. 
According to one aspect of the present invention, there is provided an 
apparatus for applying a magnetic field to an information recording medium 
and recording information comprising: 
a magnetic unit having at least first and second electromagnetic coils 
linearly arranged and separated by a gap region from each other and 
disposed oppositely to the recording medium, and 
means for selectively energizing the first and second coils in accordance 
with a region on the medium irradiated with a light beam, the first coil 
being energized to generate a first magnetic field having a first 
predetermined intensity, when a first region on the medium opposed to the 
first coil is irradiated with the light beam, the second coil being 
energized to generate a second magnetic field having a second 
predetermined intensity, when a second region on the medium opposed to the 
second coil is irradiated with the light beam, and the first and second 
coils being energized to generate a third magnetic field having a third 
predetermined intensity when a region on the medium opposed to a gap 
region between the first and second coils is irradiated with the light 
beam. 
According to another aspect of the present invention, there is provided an 
apparatus for applying a magnetic field to an information recording medium 
and recording information comprising: 
a magnetic unit having at least first and second electromagnetic coils 
linearly arranged and separated through a gap region from each other and 
disposed oppositely to the recording medium, and 
means for selectively energizing the first and second coils in accordance 
with a region on the medium irradiated with a light beam, the first coil 
being energized to generate a first magnetic field having a first 
predetermined intensity, when a first region on the medium opposed to the 
first coil is irradiated with the light beam, the second coil being 
energized to generate a second magnetic field having a second 
predetermined intensity, when a second region on the medium opposed to the 
second coil is irradiated with the light beam, and one of the first and 
second coils being energized to generate a third magnetic field having a 
third predetermined intensity larger than the intensity of the first and 
second magnetic fields when a region on the medium opposed to a gap region 
between the first and second coils is irradiated with the light beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of an apparatus having a magnetic unit for generating a 
magnetic field of the present invention will be described with reference 
to FIGS. 6 to 10. FIG. 6 shows a block diagram of a circuit for energizing 
a magnetic unit. The magnetic unit is similar to the conventional unit 
shown in FIG. 3 and no description is made here in this magnetic unit. 
In FIG. 6, an information signal detected by optical head 11 as shown in 
FIG. 1 is input through terminal 2 to signal processor 1. Processor 1 
processes the input signal and outputs a track address signal contained in 
the information signal and designating a track of optical disk 10 which is 
scanned with a laser beam emitted from head 11. Processor 1 generates a 
switching signal in response to the address signal and the switching 
signal is supplied to switching unit 3, which can generate first and 
second energizing signals in response to the switching signal to coil 
drive circuits 4A and 4B. When the first and second signals are supplied 
to coil drive circuits 4A and 4B, respectively, the coil drive circuits 4A 
and 4B supply currents to coils 18A and 18B of magnetic unit 14 to 
generate a vertical magnetic field to disk 10. In other words, coil drive 
circuits 4A and 4B are energized in accordance with a region of disk 10 
onto which the laser beam is irradiated, and coils 18A and 18B of unit 14 
corresponding to energized coil drive circuits 4A and 4B generate a 
vertical magnetic field. 
The operation of the circuit in FIG. 6 will now be described in detail. 
When a disk region Z.sub.1 is scanned with the laser beam and the track 
address signal designating the disk region Z.sub.1 is generated from 
processor 1, processor 1 generates a first switching signal to switching 
unit 3. Unit 3 generates a first energizing signal to drive circuit 4A to 
energize drive circuit 4A. Drive circuit 4A supplied with the first 
energizing signal supplies an exciting current to coil 18A of unit 14 
which generates a first vertical magnetic field H.sub.1 applied to optical 
disk 10 and having a magnetic intensity distribution as shown in FIG. 9A. 
When a disk region Z.sub.2 is scanned with the laser beam and the address 
signal designating the disk region Z.sub.2 is generated from processor 1, 
processor 1 generates a second switching signal. Unit 3 generates a second 
energizing signal to drive circuit 4B to energize drive circuit 4B. Coil 
drive circuit 4B supplied with the second energizing signal supplies an 
exciting current to coil 18B of unit 14, which generates a second vertical 
magnetic field H.sub.2 applied to optical disk 10 and having an intensity 
distribution as shown in FIG. 7B. Further, a disk region Z.sub.3 
corresponding to a region between bar section segments 17A and 17B is 
scanned with the laser beam and a track address signal designating the 
disk region Z3 is generated from processor 1, processor 1 generates a 
third switching signal to switching unit 3. Unit 3 generates first and 
second energizing signals to drive circuits 4A and 4B to energize drive 
circuits 4A and 4B. Drive circuits 4A and 4B supplied with the first and 
second signals supply exciting currents to coils 18A and 18B of unit 14, 
which generate a third vertical magnetic field (H.sub.1 +H.sub.2) applied 
to optical disk 10 and having an intensity distribution as shown in FIG. 
7C. In other words, a magnetic field which is the sum of first and second 
vertical magnetic fields is applied to disk region Z.sub.3 facing the 
space region between segments 17A and 17B. 
Since the corresponding coil is energized in accordance with the region 
irradiated with the laser beam as described above, the apparatus for 
generating a magnetic field can operate with low power consumption, can 
reduce the heat generated from the coils, and can apply a magnetic field 
having sufficient intensity for all regions scanned with the light beam on 
the information recording medium. 
Signal processor 1 in FIG. 6 may be designed, for example, in an 
arrangement as shown in FIG. 8. In processor 1, address signal detecting 
circuit 21 detects an address signal from the input signal supplied from 
head 10, and supplies the signal to ROM 22, which outputs a switching 
signal stored in the ROM in accordance with the address signal to 
switching unit 3, which thus selects either the first or second coil or 
both. As a result, the selected coil is energized. 
FIG. 9 is a block diagram showing an apparatus for generating a magnetic 
field according to another embodiment of the present invention. In FIG. 9, 
signal processor 1 is connected directly to switching unit signal 
generator 3, and connected through a drive circuit 25 to switching unit 3. 
In the circuit in FIG. 9, when a first region Z.sub.1 is scanned with a 
laser beam and the track address signal is generated from processor 1, 
driving circuit 25 is connected through switching unit 3 to first coil 18A 
in response to a first switching signal generated from processor 1, and 
driving circuit 25 generates a first energizing signal. This signal is 
supplied to coil 18A, which generates a first vertical magnetic field H1 
as shown in FIG. 10A to light disk 10 from magnetic unit 14. When a second 
region Z2 is scanned with the laser beam and the track address signal is 
generated from processor 1, drive circuit 25 is connected through 
switching unit 3 to second coil 18B in response to a second switching 
signal generated from processor 1, and driving circuit 25 generates a 
second energizing signal having a level substantially equal to that of the 
first energizing signal. This second energizing signal is supplied to coil 
18B, which generates a second vertical magnetic field H.sub.2 as shown in 
FIG. 10B to disk 10 from unit 14. Further, when a region Z3 facing a gap 
region between bar segments 17A and 17B is scanned with the laser beam the 
track address signal is produced from processor 1, driving circuit 25 is 
connected to first coil 18A through switching unit 3 in response to a 
third signal generated from the processor 1, and driving circuit 25 
generates a third energizing signal having a level larger than the first 
energizing signal. This third energizing signal is supplied to coil 18A of 
magnetic unit 14, which generates a third verticalmagnetic field H.sub.4 
having an intensity larger than the first vertical magnetic field H.sub.1 
as shown in FIG. 10C. In other words, the third magnetic field H.sub.4 
having a large intensity is applied only to third region Z.sub.3 
corresponding to the gap region between segments 17A and 17B. 
The circuit of FIG. 9 is simplified as compared with that of FIG. 6. 
In the embodiments described above, a magnetic unit having a pair of coils 
has been described. However, the present invention is not limited to the 
particular embodiments described above. For example, a magnetic unit 
having a plurality of coils, not limited to a pair of coils, may be 
applied to the present invention.